feat: add execution observability for tools, prompts, and resources

[FiammaMuscari]

Closes #460

This PR adds opt-in execution observability for tools, prompts, and resources in xmcp.

• Adds an observability config in xmcp.config.ts
• Emits structured JSON logs to stderr (safe for stdio transport)
• Logs execution.start / execution.end with duration, success, and error
• Supports optional input logging via includeInput
• Centralized implementation via execution logger
• Integrated across tools, prompts, and resources

Validation:

• tested successful and failing tool execution
• tested prompt and resource execution paths
• verified config toggling (enabled / includeInput)
• ensured no interference with stdout / MCP protocol
• fixed non-serializable input handling
• fixed non-throw tool error classification

Notes:

• circular inputs cannot be tested end-to-end due to JSON transport constraints

[greptile-apps]

PR author is not in the allowed authors list.

[vercel]

@FiammaMuscari is attempting to deploy a commit to the xmcp Team on Vercel.

A member of the Team first needs to authorize it.

[0xKoller]

Thanks for the PR @FiammaMuscari!

Looks promising overall. Before a thorough review, could you please add:

• A concrete usage example
• Screenshots
• Observability: ECS-compatible fields, OpenTelemetry-style events, W3C Trace Context + good human readability
• Sensitive key redaction
• Documentation updates

[valebearzotti]

What's the state of this PR?

[FiammaMuscari]

Hi @valebearzotti ! should I rework this PR?