feat(session): serve Anthropic Messages through SessionCore

[guapisolo]

ci-sglang-pr: #35127

Summary

Serve Anthropic Messages through Miles sessions while preserving canonical OpenAI records.

Motivation

Claude Code uses /v1/messages, while Miles session/TITO previously handled only /v1/chat/completions. Add a strict Anthropic HTTP adapter that reuses the existing SessionCore, retains canonical OpenAI records and response identity, and returns Anthropic JSON or eager fake SSE without moving session lifecycle or TITO ownership.

Usage

curl -X POST "$SESSION_SERVER/sessions/$SESSION_ID/v1/messages" \
  -H 'content-type: application/json' \
  -d '{"model":"m","max_tokens":512,"messages":[{"role":"user","content":"hi"}]}'

Design Notes

• Mental model: The route validates an Anthropic request, converts it to OpenAI form, delegates state and backend work to the existing SessionCore, stores the canonical record, then converts the committed response back to Anthropic JSON or eager fake SSE.
• Adapter boundary: anthropic_adapter.py owns parsing, fixed-feature policy, reasoning replay, and wire framing.
• Route boundary: sessions.py keeps route ordering, lifecycle, and core calls.
• Conversion ownership: SGLang PR #35127 at 1ed0c902c28ba0c5bf3c79515c940ad8c759e100 owns Anthropic/OpenAI conversion.
• Miles policy: The adapter preserves response IDs and rejects unsupported error tool results before core mutation.

Verification

• tests/fast/router/test_session_anthropic.py and tests/fast/utils/test_utils/test_mock_sglang_server.py on miles-h200-dev-2: 68 passed, 1 skipped on source 46b3db8e.
• Candidate 9d5232b3: range-diff equivalent to the tested source; upstream changed none of the five PR files.
• git diff --check c7d81d47...9d5232b3: passed.

Review Focus

• anthropic_adapter.py: policy and reasoning-history conversion boundaries.
• sessions.py: route ordering, commit semantics, and response-ID identity.