[AI-3] Probe models for embedding support automatically

[tangopium]

Ticket

AI-3

What are you trying to accomplish?

PR 8 of 11 in the AI-3 stack. The one behavioural probe worth writing: a single embeddings request per candidate model, whose 200 response also carries the vector dimension count. The background pass after a credential change is capped at ten models whose names suggest they embed, because a gateway can list hundreds and some providers bill per request. A definite refusal records unsupported, a vector records supported, and anything else records nothing, because servers silently drop unknown parameters and a 200 alone proves nothing. Administrator assertions are never overwritten.

Merge checklist

• Added/updated tests
• Behind the llm_connection feature flag

Stacked on #24885.

[github-actions]

Caution

The provided work package version does not match the core version

Details:

• Work package URL: https://community.openproject.org/work_packages/66020
• Work package version: not set
• Core version: 17.9.0

Please make sure that:

• The work package version OR your pull request target branch is correct

[github-actions]

Caution

The Enterprise plan field is not set on the work package

Details:

• Work package URL: https://community.openproject.org/work_packages/66020

Please make sure that:

• The work package Enterprise plan field is set

[chatgpt-codex-connector]

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[github-actions]

Warning

Flaky specs

• rspec ./spec/features/work_packages/table/switch_types_spec.rb[1:1:1]

🤖 Ask Copilot to investigate

Copy the prompt below into a new comment on this PR to delegate the investigation to GitHub Copilot. It will look into the flakiness and open a separate pull request with you as reviewer.

@copilot The following spec(s) are flaky in CI (first seen on PR #24886, linked for reference only):

- `rspec ./spec/features/work_packages/table/switch_types_spec.rb[1:1:1]`

Treat this as a standalone task, unrelated to PR #24886. Create a new branch from origin/dev and open a new pull request targeting dev — do not stack it on PR #24886 or reuse that branch.

Follow the playbook in docs/development/testing/handling-flaky-tests/README.md to find the root cause and fix the underlying race — do not skip, delete, or weaken the spec to make it pass; disabling is a last resort per the playbook, and only with a bug ticket. Verify the fix by running the spec(s) repeatedly (e.g. `script/bulk_run_rspec --run-count 10`).

If you cannot reproduce the flake or are not confident in a fix after reasonable investigation, do not fabricate a change or skip the spec to force CI green. Instead, leave the pull request in draft and document what you tried, the suspected cause, and any leads in its description, then assign @tangopium to take over.

Once the fix is verified, title the PR after the spec(s) it fixes, and use the PR description to explain the root cause, how the change resolves it, and the before/after results. Label the PR `flaky-spec`, assign @tangopium, and request a review from @tangopium.
On every commit, set @tangopium as the sole co-author with a `Co-authored-by:` trailer (use their GitHub no-reply email so it links to their account), so it is traceable who dispatched the fix.