feat: continue interrupted generation streams

[pjb157]

Summary

• add opt-in post-response continuation for interrupted Completions, Chat Completions, and Responses streams
• preserve protocol identity, sanitization, provider selection, and bounded retry state across continuation attempts
• document configuration and operational limits while keeping generated deployment configs disabled by default

Testing

• cargo test -p onwards --all-features
• cargo clippy -p onwards --all-targets --all-features -- -D warnings
• cargo fmt --all -- --check
• cargo check -p dwctl --lib
• cargo test -p dwctl sync::onwards_config --lib
• mdbook build onwards/docs

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[Copilot]

Pull request overview

This PR adds an opt-in “stream continuation” mechanism to Onwards so interrupted SSE text-generation streams (Completions, Chat Completions, Responses) can be resumed by issuing a follow-up request (prefix-based) to another provider in the same pool, while preserving protocol identity and keeping retries bounded and configurable.

Changes:

• Introduces stream_continuation configuration under pool fallback, including conservative defaults and eligibility checks by exact endpoint path.
• Adds a checked SSE framer (CheckedSseStream) plus parsing helpers to reliably frame events across mixed line endings and fragmentation, and to surface framing failures distinctly where needed.
• Updates routing/handler logic to support post-header continuation, composite response policy, and sanitization adjustments (including legacy completion streaming sanitization).

Reviewed changes

Copilot reviewed 13 out of 16 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
onwards/src/target.rs	Adds StreamContinuationConfig and wires it into FallbackConfig; merges response headers across pool/provider.
onwards/src/strict/handlers.rs	Routes strict endpoints through continuation-capable handler; updates SSE “data:” parsing tolerance and sanitization skip when preserving encoded streams.
onwards/src/stream_continuation.rs	Implements core continuation state machine: eligibility, event observation/rewrites, and composite stream retry loop.
onwards/src/sse.rs	Introduces CheckedSseStream, framing errors, and parse_sse_event for structured SSE parsing.
onwards/src/response_sanitizer.rs	Tightens event-stream content-type detection; adds legacy completion streaming sanitizer preserving choices[].text.
onwards/src/response_loop.rs	Minor cleanup in SSE event JSON parsing for loop deltas.
onwards/src/main.rs	Refactors tracer shutdown into a single if let ... && let Err(...) block.
onwards/src/load_balancer.rs	Adds reusable SelectionState and ProviderPool::select_next for caller-owned selection state.
onwards/src/handlers.rs	Adds continuation-aware forwarding core, composite response policy, canonical path metadata, SSE peek updates, and continuation body wrapping.
onwards/README.md	Documents stream continuation behavior, configuration, and limitations.
onwards/CHANGELOG.md	Notes new feature and the FallbackConfig API change.
onwards/Cargo.toml	Adds mime dependency.
dwctl/src/sync/onwards_config/mod.rs	Updates config conversion to set stream_continuation: None for the new field.
Cargo.lock	Lockfile update for mime.