fix: attachment notifications wording #131

[ypldan]

Applicable issues

• fixes Attached file becomes unavailable after deleting one of them #131

Description of changes

Improve how attachment metadata is communicated to the LLM agent:

• XML format for attachment notifications: Replace plain-text attachment metadata with structured <attachments> XML, making it clearer these are user-provided files (for those that are in user msgs) and easier for the LLM to parse.
• Conditional reference_url: Only include <reference_url> element when the value is present, instead of rendering None.
• Clarified available_context tool description: Explicitly state the tool is for admin-configured context only and does not cover attached files, referencing the <attachments> section.
• Expanded test coverage: Added multi-message, role-based (assistant/tool), and edge case tests for _AttachmentFilter.

Checklist

• Title of the pull request follows Conventional Commits specification
• Documentation is updated/created (if applicable)
• Changes are tested on review environment
• Integration tests pass

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[ypldan]

/deploy-review

GitHub actions run: 22577540431

Stage	Status
deploy-review	Success ✅
matrix.application	Skipped ➖

[ypldan]

/deploy-review

GitHub actions run: 22580899812

Stage	Status
deploy-review	Success ✅
matrix.application	Skipped ➖