Add bounded, validated Messages streaming and tool scheduling

[franciscojavierarceo]

Problem statement / motivation

Enterprise deployments need predictable memory use and explicit completion semantics for Messages requests that execute built-in tools. Extend the existing native Messages loop with a documented lifecycle and resource contract across all inference rounds.

Proposed solution

• Validate message and content-block transitions through one Messages ingestion path, keeping transport framing and client delivery separate from semantic validation.
• Bound retained text, thinking, signatures, tool arguments, tool call outputs, pending calls, and reconstructed history. Distinguish retained bytes from wire bytes so smaller deltas do not consume disproportionate response capacity.
• Reuse bounded scheduling contracts for built-in tools, with defined timeout, overload, disconnect, cancellation, and task-cleanup behavior.
• Preserve native Messages content, client-executed function tools, rebased indexes, and one public message lifecycle.

Acceptance criteria

• Invalid ordering, duplicate indexes/completions, missing identifiers, malformed event data, and open blocks at termination have explicit tested outcomes.
• Premature EOF cannot dispatch an incomplete tool call or become an apparently successful message. Document any intentional engine-specific tolerance; retain forward compatibility for harmless unknown events.
• Limits cover accumulated content and pending work across rounds, with deterministic errors and bounded resource release.
• Tests cover fragmented UTF-8/SSE, many small deltas, large arguments/outputs, slow readers, disconnects during inference/tools, and round-limit exhaustion.
• Preserve the message_stop completion and forced-tool-choice behavior shipped in fix: finish Messages rounds at message_stop #290 and fix: let Messages answer after forced tool use #296.

Additional context

Enterprise Readiness enhancement. Coordinate with #241/#244 and #288 for shared boundary and budget contracts; this issue owns Messages behavior. No worker-placement change is required without measurements.

Source review at afa6c14, not a new runtime reproduction: Messages streaming and Messages JSON loop.

Enterprise Readiness tracker: #316