Fix crash with unavailable WMS layers

[uclaros]

Description

Fixes #65913 which seems to be the same as #62372 which had an unsuccessful fix.

The wms provider would consider that the capabilities' mQueryableForLayer is populated even when the capabilities were unreachable/invalid.

AI tool usage

• AI tool(s) (Copilot, Claude, or something similar) supported my development of this PR. See our policy about AI tool use. Use of AI tools must be indicated. Failure to be honest might result in banning.

[github-actions]

🪟 Windows Qt6 builds

Download Windows Qt6 builds of this PR for testing.
To execute locally, unzip the downloaded zip file and run bin\qgis-bin.exe in the extracted directory.
You might be prompted by Windows Defender click "Run anyway"
(Built from commit 8b879c8)

[qgis-bot]

The backport to release-3_44 failed:

The process '/usr/bin/git' failed with exit code 1

stderr

error: could not apply 8b879c8ff31... Only check if layer is queryable when capabilities are valid
hint: After resolving the conflicts, mark them with
hint: "git add/rm <pathspec>", then run
hint: "git cherry-pick --continue".
hint: You can instead skip this commit with "git cherry-pick --skip".
hint: To abort and get back to the state before "git cherry-pick",
hint: run "git cherry-pick --abort".
hint: Disable this message with "git config set advice.mergeConflict false"

stdout

Auto-merging src/providers/wms/qgswmsprovider.cpp
Auto-merging tests/src/providers/testqgswmsprovider.cpp
CONFLICT (content): Merge conflict in tests/src/providers/testqgswmsprovider.cpp

To backport manually, run these commands in your terminal:

# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add .worktrees/backport-release-3_44 release-3_44
# Navigate to the new working tree
cd .worktrees/backport-release-3_44
# Create a new branch
git switch --create backport-66578-to-release-3_44
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick 8b879c8ff31cf4bf655ce54a65effdf59a3e993b
# Push it to GitHub
git push --set-upstream origin backport-66578-to-release-3_44
# Go back to the original working tree
cd ../..
# Delete the working tree
git worktree remove .worktrees/backport-release-3_44

Then, create a pull request where the base branch is release-3_44 and the compare/head branch is backport-66578-to-release-3_44.