fix(api-keys): keep assigned account picker from closing dialog

[laaichiu]

Set the assigned-account dropdown to non-modal mode so its portaled menu does not trigger the parent API key dialog's outside-dismiss handling when focus returns to the form.

Update the edit-dialog regression test to select accounts and click another in-dialog control before saving, proving the draft remains saveable without pressing Escape first.

Summary

Type of change

• fix: — bug fix (no behavior change beyond the bug)
• feat: — new user-facing feature or capability
• refactor: — internal refactor (no behavior change, no API change)
• docs: — documentation only
• chore: / ci: / build: — tooling, CI, packaging
• test: — test-only change
• Breaking change (also append ! after the type, e.g. feat!: or include BREAKING CHANGE: footer)

Linked issue:

OpenSpec

• This PR includes / updates an OpenSpec change
• Not applicable — bug fix that matches the existing spec
• Not applicable — docs / CI / chore only
• This PR touches a codex-faithful path (image pipeline, request/response
  shape, SSE framing, OAuth flow) and preserves upstream-equivalent behavior

Change directory:

Changes

Test plan

# uv run pytest tests/unit/test_<area>.py -q
# uv run pytest tests/integration/test_<area>.py -q

Screenshots / output (optional)

Checklist

• Title is in Conventional Commits format (<type>(<scope>)?: <subject>).
• Linked the related issue / discussion above.
• Added or updated tests covering the change.
• Ran uv run pre-commit run local-ci --hook-stage manual --all-files or the relevant make <target> subset locally.
• If touching specs: openspec validate --specs passes and /opsx:verify is clean.
• CHANGELOG is not edited by hand (release-please handles it).