fix: accept SSE data lines without a space after the colon

[CodePrometheus]

Description

The space after data: is optional per the SSE specification, but the translators only matched the spaced form, so a stream sending data:{...} had every event silently skipped.

The response bytes still reach the client untouched, but the gateway observes nothing: token usage, the resolved response model, and tracing chunks are all lost. In production this looks like requests succeeding while usage is recorded as zero, which breaks usage-based metrics and any downstream cost or quota logic.

This adds cutSSEFieldPrefix, which strips an SSE field name and removes at most one leading space, and uses it at every data: read site plus the event: read site in the Anthropic stream parser. Emitted streams are unchanged and keep the canonical spaced form.

Related Issues/PRs (if applicable)

Related to #2149

Special notes for reviewers (if applicable)

The new streaming regression tests fail without the fix: usage comes back all zeros while the response itself passes through normally.

[dosubot]

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 93.93939% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.05%. Comparing base (b80bc3e) to head (207fe34).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/translator/openai_responses.go	50.00%	0 Missing and 1 partial ⚠️
internal/translator/openai_speech.go	50.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2470      +/-   ##
==========================================
+ Coverage   85.03%   85.05%   +0.02%     
==========================================
  Files         159      159              
  Lines       22634    22645      +11     
==========================================
+ Hits        19247    19261      +14     
+ Misses       2219     2217       -2     
+ Partials     1168     1167       -1     

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[mathetake]

/retest