What does this PR do?

extract_media() fails to deliver a file when the model wraps the delivery tag
in Markdown emphasis — **MEDIA:/path.pptx**, *MEDIA:/path*,
_MEDIA:/path_. The file is then silently never attached and the literal
MEDIA:/path text is shown to the user instead (the reported symptom: "the bot
sends a path instead of the file").

Root cause is purely in MEDIA_TAG_CLEANUP_RE (gateway/platforms/base.py):

• the leading anchor only tolerated a single quote/backtick ([`"']?), so
  a leading **/*/_ prevented the match;
• the closing lookahead set (?=[\s"',;:)]}]|$)excludedand_, so a trailing **//_` also prevented the match.

This change lets a short run of emphasis/quote markers ([`"'*_]{0,3})
appear on either side of the tag and adds */_ to the closing lookahead. It
keeps the existing strict behavior (a recognized extension is still
required — no loose \S+ fallback) and does not touch the
_mask_protected_spans / _mask_json_string_media layer, so the deliberate
suppression of example tags inside code blocks, inline code, blockquotes and
serialized JSON (#35695, #34375) is preserved. The absolute-path anchor
(incl. the Windows drive support from #34632) is unchanged, so relative paths
are still rejected and _ inside a filename is unaffected.

Observed in production across multiple Telegram instances: on instances where
the model emitted emphasis-wrapped tags, those file deliveries were lost 1:1;
instances that never wrapped the tag had zero loss. Some Grok/GPT variants bold
file names by default, so the collision is frequent.

Related Issue

Fixes #23759.

Supersedes #23765 (open since 2026-05-11, no reviews): that patch predates the
refactor that moved the inline regex into the module-level
MEDIA_TAG_CLEANUP_RE, switched the hardcoded extension list to
_MEDIA_EXT_ALTERNATION, added the Windows anchor (#34632) and the masking
layer (#35695). It also reintroduced the loose |\S+ path fallback that the
refactor had removed. This PR applies cleanly on current main, is more
general (handles mixed quote+emphasis and __/***), preserves strict
extension matching, and adds masking-invariant regression coverage. Credit to
@LeonSGP43 for the original report and fix.

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)

Changes Made

• gateway/platforms/base.py — MEDIA_TAG_CLEANUP_RE: tolerate
  [`"'*_]{0,3} on both sides of the tag and add */_ to the closing
  lookahead. Both the non-streaming dispatch path and the streaming consumer
  reference this constant, so extraction and visible-text stripping are fixed
  on both paths at once.
• tests/gateway/test_platform_base.py — added TestExtractMedia cases for
  bold/italic(*)/italic(_) wrapping, mid-prose bold, emphasis-wrapped
  .html, underscore-in-filename (must be unaffected), and
  emphasis-wrapped relative path (must stay rejected).

How to Test

1. Before: BasePlatformAdapter.extract_media("**MEDIA:/tmp/r.pptx**") → ([], ...) (file dropped, literal text leaks).
2. After: same input → ([("/tmp/r.pptx", False)], "") (delivered, text stripped).
3. pytest tests/gateway/test_platform_base.py -q → all pass (incl. the
   pre-existing code-block / inline-code / blockquote / JSON suppression tests,
   confirming no regression of Bug: extract_media() false-positives on example paths in quoted text / code blocks #35695 / Gateway MEDIA extraction can attach stale files from serialized tool/search-result text #34375).

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(gateway):)
• I searched for existing PRs (found fix(gateway): strip markdown wrappers from media paths #23765 — see Related Issue; this supersedes it)
• My PR contains only changes related to this fix
• I've run pytest tests/ -q and all tests pass — ran the relevant file tests/gateway/test_platform_base.py (165 passed, 2 skipped); change is an isolated regex edit
• I've added tests for my changes
• I've tested on my platform: Debian 13 (and the regex is platform-agnostic)

Documentation & Housekeeping

• I've updated relevant documentation (docstring above the regex) — code comment added
• N/A — no config keys changed
• N/A — no architecture/workflow change
• I've considered cross-platform impact — the Windows drive anchor (Bug: MEDIA directive silently fails on Windows #34632) is preserved; emphasis markers are ASCII
• N/A — no tool behavior/schema change