Extract SSE byte buffer policy

[JerrettDavis]

Description

Extracts the pure SSE byte-buffer parser from helpers.py into headroom.proxy.sse_byte_buffer_policy. Existing helper imports remain as delegates, while the protocol parser now has its own module and direct tests.

Closes #

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Performance improvement
• Code refactoring (no functional changes)

Changes Made

• Added sse_byte_buffer_policy.py for SSE terminator detection and complete-event parsing.
• Kept helpers.parse_sse_events_from_byte_buffer and _find_sse_event_terminator delegating to the extracted policy.
• Added direct policy tests for LF/CRLF terminators, buffer draining, split UTF-8 preservation, and invalid complete UTF-8 events.
• Carried forward the LiteLLM callback compatibility shim needed for current mypy on main.

Testing

• Unit tests pass (pytest)
• Linting passes (ruff check .)
• Type checking passes (mypy headroom)
• New tests added for new functionality
• Manual testing performed

Test Output

python -m pytest tests\test_sse_byte_buffer_policy.py tests\test_sse_utf8_split.py
8 passed in 0.23s

python -m ruff check .
All checks passed!

python -m ruff format --check .
1095 files already formatted

python -m mypy headroom --ignore-missing-imports
Success: no issues found in 409 source files

gitleaks protect --staged --no-banner --redact
no leaks found

Real Behavior Proof

• Environment: Windows, Python 3.13.13, branch jd/architecture-slice-29.
• Exact command / steps: ran new SSE byte-buffer policy tests, existing SSE UTF-8 split tests, ruff, ruff format check, mypy, and staged gitleaks scan.
• Observed result: SSE parser behavior remains covered and local lint/type/security checks pass.
• Not tested: live streaming proxy runtime; existing helper imports remain intact.

Review Readiness

• I have performed a self-review
• This PR is ready for human review

Checklist

• My code follows the project's style guidelines
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated the CHANGELOG.md if applicable

Screenshots (if applicable)

N/A

Additional Notes

Documentation and changelog updates are N/A for this internal architecture-only refactor. The push reported existing default-branch Dependabot alerts; no staged secret leaks were found for this PR.

[github-actions]

PR governance

This PR follows the template and is marked ready for human review.