fix(gateway): allow [[as_document]] glued directly to MEDIA path

[euisuh]

What does this PR do?

Allows a recognized MEDIA: path to be followed immediately by [[as_document]]. The cleanup/extraction matcher previously rejected [ as a post-extension delimiter, so the media tag was left in text and no attachment was extracted.

The strict media-tag matcher remains unchanged; [ is accepted only after an already-recognized media extension.

Related Issue

Fixes #63632

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✅ Tests (adding or improving test coverage)

Changes Made

• Add [ to the known-extension delimiter lookahead in gateway/platforms/base.py.
• Add a regression covering extraction and exact cleanup of a glued [[as_document]] directive.

How to Test

1. Run scripts/run_tests.sh tests/gateway/test_platform_base.py -q.
2. Confirm all 193 tests pass.
3. The new regression verifies MEDIA:/tmp/report.xlsx[[as_document]] extracts /tmp/report.xlsx and cleans the surrounding text exactly.

Checklist

Code

• I've read the Contributing Guide
• My commit message follows Conventional Commits
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix
• I've run the entire repository test suite (targeted suite: 193 passed)
• I've added tests for my changes
• I've tested on macOS

Documentation & Housekeeping

• Documentation update: N/A
• Config example update: N/A
• Architecture/workflow docs update: N/A
• Cross-platform impact considered; regex behavior is platform-independent
• Tool descriptions/schemas update: N/A

[alt-glitch]

This was generated by AI during triage.

Duplicate of #63644: both PRs implement the same focused MEDIA cleanup-regex delimiter change for glued [[as_document]] directives.

[euisuh]

Closing as a duplicate of #63644, which already implements the same cleanup-regex delimiter fix and has green CI. I missed it during initial issue triage.