refactor: unify response processing under AgentPipeline

[maralbahari]

Summary

The Responses executor previously split JSON/SSE accumulation, tool-call translation, and stream emission across upstream.rs, accumulator.rs, and function_sse.rs. This made ownership unclear and allowed live and collected responses to follow different processing paths.

This PR implements #243 from RFC #241 by introducing a request-owned AgentPipeline:

• Route complete JSON and live SSE bodies through the same pipeline and finalization path.
• Add a synchronous, round-scoped RoundIngestion that combines typed response accumulation with tool-call translation.
• Split ResponseAccumulator into focused modules with typed active/completed slots keyed by validated output_index.
• Apply strict and lenient validation policies through the same state transitions.
• Replace the monolithic function SSE translator with an inline dispatcher and tool-specific translators for function, custom, namespace, and tool-search calls.
• Keep ToolRegistry as the request-scoped lookup for available tools and their client/gateway ownership. Translators consume an owned classification snapshot.
• Keep gateway-owned tool execution and public lifecycle generation in the gateway path.
• Move ordered, awaited stream delivery into AgentPipeline, preserving cross-round sequencing, output-index rebasing, deferred event ordering, and downstream backpressure.
• Encapsulate the tool loop, registry, response budget, and pipeline under EngineOrchestration.
• Keep upstream.rs focused on request serialization, inference transport, response-budget accounting, and selecting the JSON or SSE pipeline runner.
• Document the final ownership boundaries, canonical tool normalization, client translation, and OutputItem::to_input_item as the continuation conversion policy.
• Refresh the gateway replay cassettes for custom tools, MCP, parallel tool calls, tool search, and web search.

Closes #243.
Part of #241.

Test Plan

• cargo test --locked --workspace -- --test-threads=2
  • 1120 passed, 0 failed, 9 ignored
• cargo clippy --locked --workspace --all-targets -- -D warnings
• git diff --check
• Re-recorded and replayed the affected gateway cassettes.

[franciscojavierarceo]

Reviewed ingestion, streaming delivery, tool translation, and persistence boundaries at e03fb7e. No actionable regressions found. Validation: 877 core tests and 19 split-execution tests passed; 9 tests were ignored. Cassette replay coverage passed; live providers were not tested.