fix(core): fail empty provider output

[rasitakyol]

Issue for this PR

Closes #37372

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Treats a successful provider stream with no visible text or tool call as provider.invalid-output. This prevents empty or reasoning-only responses from ending the execution as a silent success.

The test fixtures that intentionally represent successful turns now include usable text or a completed tool call.

How did you verify your code works?

• Ran bun test test/session-runner.test.ts test/session-execution.test.ts --only-failures: 135 tests passed.
• Ran bun typecheck in packages/core.
• Passed the pre-push workspace typecheck: 32/32 tasks.
• Ran targeted Oxlint and git diff --check.

Screenshots / recordings

Not applicable; this changes the V2 provider-completion contract and tests.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

[rasitakyol]

The linked issue is #37372 and the PR body contains Closes #37372. This change targets v2 because the affected Session V2 implementation is not present on dev. GitHub does not populate closingIssuesReferences for a PR whose base is not the default branch, so the standards workflow added needs:issue despite the explicit reference. Retargeting to dev would include the unrelated divergent v2 branch. Maintainers: please confirm this is the preferred base and remove the false-positive label, or advise the expected linkage for v2 PRs.