Add bounded, validated Messages streaming and tool scheduling

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 #290 and #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: https://github.com/vllm-project/agentic-api/issues/316

Start by reading crates/agentic-server-core/src/executor/messages_stream.rs and messages_loop.rs at commit afa6c14, then review the shared boundary and budget contracts in issues #241, #244, and #288. Define the Messages lifecycle and resource behavior before implementing it; done means the listed ordering, EOF, limit, cancellation, streaming, tool, and round-limit cases have deterministic tests while preserving message_stop and forced-tool-choice behavior.