test: stop example tests from writing SQLite files into examples/

[dgageot]

Some examples (memory, RAG, response cache) define toolsets with relative paths
like `./funny.db`, `./bm25.db`, `./cache.json`. When `TestLoadExamples` ran,
the loader resolved those against the example file's parent directory, so the
test was creating real SQLite/JSON files inside the `examples/` tree.

Fix: in the test, wrap the example bytes in a `config.NewBytesSource` (whose
`ParentDir()` returns `""`) and set `runConfig.WorkingDir` to a per-subtest
`t.TempDir()`. The existing `cmp.Or(agentSource.ParentDir(), runConfig.WorkingDir)`
fallback in the loader then redirects every relative path into the temp dir.

Bonus: since each subtest now has its own temp dir, the previous SQLite-lock
contention between examples that share relative DB names is gone, so the
subtests run in parallel.

A second commit tidies up `TestLoadExamples` (extracts an env-gathering
helper, inlines the anonymous `versioned` struct, drops a redundant
`if err != nil` wrapper around an `errors.Is` check, and flattens the
`RuntimeConfig` literal).