fix: enrich file attachment fallback context

[iuiu-py]

Motivation

When file extraction is disabled, the file fallback only tells the model about the downloaded local path. The original file URL is lost, and the model is not explicitly told how to inspect the downloaded attachment. This makes it harder for models to handle file-only messages. Fixes #8997.

Changes

• Added a shared formatter for normal and quoted file attachment fallback text.
• Included the source URL when the platform provides one.
• Told the model to use astrbot_file_read_tool with the local path when that tool is available.
• Kept the existing path-only fallback format for attachments without a URL.
• Applied Ruff formatting to an existing long test signature so the format check passes.

Testing

• Added regression tests for normal and quoted file-only messages, with and without a URL.
• uv run pytest tests/unit/test_astr_main_agent.py -q - 143 passed.
• uv run ruff format --check .
• uv run ruff check .

Summary by Sourcery

Improve file attachment fallback messages so models retain source context and know how to inspect downloaded files.

Bug Fixes:

• Enrich file attachment fallback context with the source URL when available and guidance for inspecting the downloaded file with the file-reading tool.

Enhancements:

• Use a shared formatter for normal and quoted file attachment fallback text while preserving path-only output when no URL is provided.

Tests:

• Add regression coverage for normal and quoted file-only messages with and without source URLs.

[sourcery-ai]

Hey - I've reviewed your changes and they look great!

Sourcery assessment

Approved.

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