fix: bound incoming transport message sizes

[benbrandt]

Summary

Bound incoming message assembly for NDJSON and HTTP/SSE transports while retaining enough default headroom for multimodal ACP prompts.

Changes

• Add an optional maxMessageBytes setting with a 32 MiB default, plus shared DEFAULT_MAX_MESSAGE_BYTES and MessageTooLargeError exports for the stable, v2, and HTTP entry points.
• Replace retained chunk arrays with a bounded, geometrically growing byte buffer and yield completed lines lazily.
• Bound both unfinished SSE lines and accumulated event data. Discard comments and unused fields immediately, account for framing separately, and reset the payload budget per event.
• Clear buffered data, cancel the input, and error the connection on overflow rather than silently dropping requests or responses.
• Bound HTTP initialization JSON and error response bodies, and cancel unused successful POST/DELETE response bodies without waiting indefinitely for cleanup.
• Preserve existing below-limit parsing behavior, including UTF-8 chunk boundaries, BOM handling, CRLF, multiline SSE, and accepted EOF-terminated messages.

Compatibility and scope

• Existing call sites remain valid, but the finite default intentionally rejects previously accepted oversized input. Callers can supply a larger positive safe-integer limit where required.
• The limit controls incoming encoded data, not outgoing serialization or peak process memory. Oversized input terminates the connection.
• The 32 MiB default accommodates more image-heavy prompts: three 4 MiB images already exceed a 16 MiB frame after base64 encoding and JSON overhead.
• The Node HTTP server's existing 16 MiB maxRequestBodyBytes default is unchanged. Deployments accepting larger HTTP prompts must configure that server-side setting separately.
• This change bounds individual message assembly. Completed-message queue backpressure, aggregate connection budgets, and WebSocket limits remain separate work.

Validation

• CI=1 npm run check passed: schema generation checks, lint, formatting, spelling, build, 886 test executions including source/compiled copies, and TypeDoc verification.
• 2,172 bounded differential parsing cases matched the original implementation's messages, protocol-error responses, and warning counts below the limit.
• Bounded probes verified exact limits, UTF-8/CRLF/BOM accounting, multiline SSE limits, cancellation, and default/override propagation.
• A synthetic prompt sized for three 4 MiB images succeeds with the default and fails with a 16 MiB override; unterminated input cancels on the first 1 MiB chunk past 32 MiB.
• Successful HTTP response cleanup was checked with resolving, rejecting, stalled, already-errored, and absent bodies.
• Node stream-adapter and loopback HTTP/SSE checks verified underlying source shutdown and connection cleanup.

No test or documentation files were added; the additional cases were exercised with bounded inline probes.