Bound buffer writes in JSON assembly and staging

[jfallows]

Fixes #2017

Description

This change hardens buffer overflow protection in the MCP HTTP binding's JSON response assembly and reply staging logic by:

1. Adding bounds checking to JSON string escaping: Modified putEscaped() to stop writing escape sequences before they would overflow the destination buffer's capacity. Added putBounded() helper for single-byte writes that respect buffer limits, and escapedWidth() to predict escape sequence lengths. This prevents pathological inputs (e.g., control characters that expand 6x when escaped) from overflowing fixed-size buffers.

2. Making staging operations return success/failure: Changed stage() and stageBytes() methods to return boolean indicating whether bytes were successfully staged or if the pooled buffer was exhausted. Callers now check the return value and stop staging further fragments once a write fails, preventing cascading writes to a released slot.

3. Propagating traceId through staging calls: Updated all staging methods to accept traceId parameter so they can properly call cleanup(traceId) when the pool is exhausted, ensuring correct resource cleanup and stream termination.

4. Updating JSON generator plain write methods: Modified write(CharSequence), writeKey(CharSequence), and writeNumber(CharSequence) overloads to route through the bounded/Completion path, allowing them to truncate cleanly and report partial consumption via consumed() instead of overflowing.

5. Adding comprehensive unit tests: New test class McpHttpProxyFactoryTest validates that buffer writes stop at capacity boundaries, escape sequences don't split across limits, and partial writes can be resumed.

The changes ensure that even when upstream responses or tool outputs contain pathological content, the binding cannot overflow its fixed-size buffers—writes simply truncate at the boundary and the stream terminates cleanly.

Test Plan

Added unit tests in McpHttpProxyFactoryTest covering:

• Buffer overflow prevention when escaped input exceeds capacity
• Correct escaping when output fits capacity
• Stopping at capacity boundary without splitting escape sequences
• Bounded single-byte writes
• Escaped width calculations

Existing JSON generator tests updated to verify partial consumption reporting and resumable plain writes.

https://claude.ai/code/session_01Nf6gRQmSySSoDLB9pcRW5M

[jfallows]

LGTM