fix: fall back when provider result is empty

[FrundlesTian]

Description

Fixes the empty response shown with OpenRouter and other Anthropic-compatible providers that return ResultMessage.result == "" while still emitting the real response in an AssistantMessage.

The result is now stripped once and used only when it contains text. Empty and whitespace-only provider results take the existing AssistantMessage extraction path; non-empty ResultMessage content keeps its current priority.

This is stacked on #236 because that PR currently owns the same result-extraction block. The only diff relative to #236 is this fix, its regression tests, and the changelog entry. After #236 merges, this PR can be retargeted to main without carrying unrelated commits.

Related issue

Closes #171

Type of change

• Bug fix
• New feature
• Breaking change (documented in CHANGELOG under "Changed" or "Removed")
• Documentation or tooling only

How it was tested

• Tests added or updated

• Equivalent make test and formatting/lint commands pass locally

• Tested by hand against a running bot

• pytest --no-cov — 685 passed

• black --check src tests

• isort --check-only src tests

• flake8 src tests

The regression test covers both an empty string and a whitespace-only result while an AssistantMessage contains the OpenRouter response.

Checklist

• One concern per PR; unrelated changes are split out
• CHANGELOG.md has an entry under [Unreleased]
• If pyproject.toml dependencies changed, poetry lock was run and the updated poetry.lock is committed (not applicable)
• Documentation updated (README.md, docs/, .env.example, CLAUDE.md) where settings or commands changed (not applicable)
• New settings default to current behaviour (not applicable)
• If AI tools helped write this change, I reviewed every line and the hand-testing above is mine

[github-actions]

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