refactor(streaming): adopt shared Praxis SSE codec for Responses framing

[leseb]

What

Migrate the OpenAI Responses provider-neutral SSE record framing onto the shared praxis_filter::sse codec introduced by praxis-proxy/praxis#1060 (epic praxis#985). SseFrameParser now delegates byte-level line/record framing to SseDecoder while keeping the OpenAI-facing SseFrame / SseParseError surface, the [DONE] sentinel, the event-count budget, timeouts, and lifecycle rules in the Responses consumers unchanged.

Praxis has no release carrying the codec yet, so the praxis core crates are pinned to main at the #1060 merge commit (c4cab79) and Pingora moves to the 0.8.4 fork in lockstep with core.

Scope — migrate only where framing matches the shared contract

• OpenAI Responses (openai_stream_events, responses::parser) → migrated onto the codec. parse_chunk now takes &Bytes and threads it into the decoder without copying.
• A2A route-capture scanner + the token-usage counter that reuses it → intentionally not migrated. They are fail-open and recoverable on an oversized record (drop it, resync at the next blank-line boundary), whereas the shared decoder poisons permanently. Adopting the codec there requires a recoverable/resync-on-overflow mode in praxis#986, not another AI-local parser. The gap is documented in the A2A sse module and referenced from the token-usage consumer.
• Anthropic anthropic_stream_events → deferred to a follow-up PR.

Behavior preservation

max_buffer_bytes maps onto both codec framing bounds (max_line_bytes and max_record_bytes). For the single-data:-line blocks the Responses API emits, the overflow trip point and peak retained bytes match the pre-codec parser; the new() doc describes the mapping truthfully (including how a multi-line record — which the Responses API never emits — is accounted differently). No full-body buffering is introduced and backpressure is not weakened for real traffic.

Duplicate pure-codec framing tests are removed in favor of adapter-focused tests plus the codec's own conformance suite.

Testing

• make lint ✓
• make test ✓
• make doc ✓ (verifies the new intra-doc links)

Refs #842

[leseb]

Draft since we need a new core release with the SSE codec code.