refactor(session): improve testability and simplify the session package

[dgageot]

Summary

Improves the testability and readability of pkg/session/ without changing any behaviour observable by callers. The branch is structured as 9 small, focused commits — each with its own validation — so it can be reviewed (and reverted) one piece at a time.

Why

The session package was hard to test in isolation:

• session.New(), UserMessage(), SystemMessage(), ImplicitUserMessage() all called time.Now() and uuid.New() directly, so tests had to overwrite CreatedAt/ID after construction or rebuild chat.Message literals by hand.
• The token-budget heuristic in truncateOldToolContent was an inline len(s)/4, so the test had to encode the divisor in comments.
• NewSQLiteSessionStore only accepted a path, forcing every store test to allocate a temp directory and write WAL files.
• MigrationManager baked in the production migration list, so any test of ordering, idempotency, version-mismatch or per-step failure had to drag in all 21 migrations.
• AddSession, UpdateSession and addSessionTx each had their own copy of the JSON marshalling for Permissions / AgentModelOverrides / CustomModelsUsed plus the parent_id NULL dance — adding a column meant editing 3+ places.
• Two near-identical message deep-copy implementations existed, and one of them (in branch.go) silently forgot to duplicate MessagePart.File, so branched sessions shared that pointer with the parent.
• scanSession read 7 columns as strings and then strconv.Parse{Bool,Int,Float}'d them.
• GetSession/loadSessionWith and loadSessionItems/loadSessionItemsWith were thin wrapper pairs, and GetMessages/GetAllMessages duplicated the same lock-snapshot-clone pattern (and disagreed on which Item fields to preserve).

What

Testability (commits 1–5)

#	Commit	Effect
1	test(session): inject clock and ID generator for deterministic tests	Package-level seams nowFn/newIDFn plus setNowForTest/setIDForTest test helpers. Constructors now produce deterministic CreatedAt/ID. Persisted migrations.applied_at keeps using real time.Now().
2	refactor(session): extract approximateTokens helper	The len(content)/4 heuristic becomes a named function with its own table-driven test; doc comments updated.
3	feat(session): add NewSQLiteSessionStoreFromDB for in-memory tests	New constructor accepts a caller-supplied *sql.DB. Tests can use sql.Open("sqlite", ":memory:") instead of needing a temp dir & WAL files. Shared setupAndMigrate helper is the new bootstrap path.
4	refactor(session): make MigrationManager migration list injectable	NewMigrationManager(db) keeps the production list (callers unchanged); NewMigrationManagerWithMigrations(db, migrations) lets tests drive synthetic migrations. Adds 7 unit tests covering rollback, retry, UpFunc failure, and version mismatch — paths that were previously unreachable.
5	refactor(session): collapse duplicated session marshalling/SQL	sessionPersistedFieldsOf(*Session) is now the single source of truth for JSON-bearing columns + parent_id NULL handling. sessionSelectColumns is the canonical SELECT list. ~30 fewer lines in store.go; future column additions become one-place changes.

Simplification (commits 6–8)

#	Commit	Effect
6	refactor(session): consolidate the two deep-copy implementations	One cloneMessage/cloneChatMessage pair instead of two. Bonus: fixes a latent bug — branch.go's version forgot to deep-copy MessagePart.File, leaving branched sessions sharing that pointer with the parent. Added a regression test that mutates the branched copy and asserts the parent is untouched.
7	refactor(session): scan SQL columns into native Go types	Modernc/sqlite returns INTEGER/BOOLEAN as their native Go types; the strconv.Parse* layer (and 7 untestable error branches) goes away. scanSession drops from ~95 to ~55 lines.
8	refactor(session): extract snapshot helper and collapse store loaders	One Session.snapshotItems() for GetMessages/GetAllMessages (which now agree on which Item fields to preserve). One loadSession(ctx, q, id) and one loadSessionItems(ctx, q, sessionID) instead of two thin-wrapper pairs.

Review polish (commit 9)

#	Commit	Effect
9	fix(session): plug review nits in test helpers	Defensive t.Cleanup(db.Close) in openMemoryStore so a migration-time failure can't leak the connection. Doc comments on setNowForTest/setIDForTest calling out that they mutate package-level state and must not be combined with t.Parallel().

Diffstat

 pkg/session/branch.go                      |  56 +---
 pkg/session/branch_test.go                 |  41 +++
 pkg/session/clock_test.go                  | 135 +++++++++
 pkg/session/migrations.go                  |  29 ++-
 pkg/session/migrations_unit_test.go        | 170 ++++++++++++
 pkg/session/session.go                     |  85 +++---
 pkg/session/store.go                       | 404 ++++++++++++-----------------
 pkg/session/store_memory_test.go           |  74 ++++++
 pkg/session/store_persisted_fields_test.go |  50 ++++
 pkg/session/truncate_test.go               |  22 ++
 10 files changed, 732 insertions(+), 334 deletions(-)

No behaviour change

• No public API removed. New constructors / helpers are additive: NewSQLiteSessionStoreFromDB, NewMigrationManagerWithMigrations, approximateTokens, sessionPersistedFieldsOf, Session.snapshotItems.
• All previous tests pass unchanged.
• One pre-existing bug fixed (the missing MessagePart.File deep-copy on branched sessions), with a regression test.

Validation

• mise lint (golangci-lint + custom lint + go mod tidy --diff): 0 issues
• go test ./... across the whole repo (~125 packages, including all consumers of pkg/session): all green
• go test -race ./pkg/session/...: green (run multiple times)
• go vet ./...: clean

How to review

The commits are deliberately self-contained and stand on their own — reviewing them in order tells the story:

1. Start with commits 1–2 (clock seam + token helper) — small, low-risk seams.
2. Commit 3 introduces the in-memory test path; commits 4–5 broaden it to migrations and SQL marshalling.
3. Commit 6 is the only one that fixes a behaviour bug (with the regression test that locks it in).
4. Commits 7–8 are pure code-shape simplification.
5. Commit 9 is review polish.