feat(mcp): instrument the remote MCP tool-dispatch chokepoint with PostHog events

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Context

Part of #6228. With the remote-side telemetry wrapper module in place (companion issue), this issue wires it into src/mcp/server.ts's actual tool-dispatch path.

Requirements

• Find (or, if one doesn't cleanly exist, create) a single chokepoint in src/mcp/server.ts through which every registered tool's invocation passes, and call the telemetry wrapper's recordMcpToolCall there — one instrumentation point, not one per tool.
• Record: tool name, caller type ("remote"), success/failure, and coarse latency (start-to-finish duration) — nothing else, per Spec: usage-telemetry instrumentation strategy for MCP (PostHog) #6228's field allowlist.
• Must not change any tool's actual behavior or response shape — this is pure observability, zero functional change.
• Must not throw or block a tool response if the telemetry call fails — telemetry failures are silently swallowed (matching the wrapper module's own no-op guarantee).

Test Coverage Requirements

99%+ Codecov patch coverage; a regression test confirming a tool call still succeeds and returns its normal response even if the telemetry call fails/throws.

Deliverables

• Every tool invocation on the remote server triggers exactly one recordMcpToolCall.
• Zero behavior change to any tool's actual response.

Expected Outcome

Real usage data starts flowing for every MCP tool call made against the remote server.

Links & Resources

• Spec: usage-telemetry instrumentation strategy for MCP (PostHog) #6228 (parent spec)
• The remote-side telemetry wrapper module issue (provides recordMcpToolCall)

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JSO-4325