refactor(gateway): extract compression-status helpers from run.py (slice 4 of #54962)

[andrexibiza]

Related #52995 #54962 #55138 #69550 #77433 #77438

What does this PR do?

Fourth slice of the Extract Gateway Platform Routing from
gateway/run.py refactor (#54962, #55138) — follow-up to #77433 (slice 1:
display helpers) and #77438 (slices 2-3: message/config + resume/replay
helpers).

This slice pulls the compression-status matcher out of the god-file
into gateway/status_helpers.py:

• _status_template_to_regex — compiles a compression status template
  constant into regex source (literal text escaped verbatim, {field}
  placeholders → numeric pattern)
• _COMPRESSION_PROGRESS_STATUS_RE — the compiled opt-in matcher,
  built from the SAME 8 template constants the emit sites format
  (agent/conversation_compression.py), so wording drift cannot
  silently diverge from the gateway matcher (fix(gateway): suppress routine pre-API compression chatter on chat platforms #69550, Opt-in progress notice during long auto-compression pauses? (compression is silent by design today) #52995)

No behavior change: byte-identical extraction (AST-verified against
origin/main). gateway/run.py shrinks by 47 lines; the 8 now-unused
template imports are removed from run.py and live only in the new module.
The matcher is imported at the extraction point so the line-628 call site
and existing tests stay green.

Scope honesty

The full platform-routing extraction from a 26.7K-line file is a
multi-PR effort. This is slice 4, using the same verified pattern as
slices 1-3 (module + import + shrink + AST-fidelity). Platform adapters
and the dispatch loop remain for follow-up slices.

How to test

pytest tests/gateway/test_compression_progress_notices.py -q
# plus regression: test_replay_entry_fields, test_restart_resume_pending
# 78 passed total

What platforms were tested?

• Windows 11 native: both files parse, imports resolve, _status_template_to_regex
  AST-identical to origin/main, 78 tests pass, git diff --check clean,
  attribution audit clean.

Why this matters

Every slice shrinks the largest file in the codebase and gives extracted
helpers direct coverage. gateway/run.py is down ~360 lines across
slices 1-4, with more to come.

Part of #54962
Part of #55138 (Extract Gateway Platform Routing)

• Refactor (no behavior change)
• Bug fix
• Breaking change

Checklist

• Code follows repo style (extraction, no new deps)
• Self-review complete
• Existing tests cover the extracted helpers (78 pass)
• git diff --check clean
• Attribution audit clean

Part of #78647