feat(server): add workload governance for timeout, cancellation, and quotas

[ganeshwhere]

Summary

This PR adds workload-governance controls to the server so long-running or abusive queries can be bounded and interrupted cleanly instead of degrading overall stability.

What Changed

• add statement timeout propagation through planner and executor execution paths
• add cooperative cancellation checkpoints across scan, filter, projection, sort, join, update, and delete flows
• add active statement tracking plus server APIs to list and cancel running statements
• add CLI support for active statement inspection and cancellation via \queries and \cancel
• enforce per-identity concurrent query quotas in the server session layer
• enforce query result byte limits in addition to existing row-count limits
• add runtime/admin counters for quota rejections, timed-out requests, canceled requests, and active statements
• add integration coverage for timeout, cancellation, quota rejection, and cleanup of canceled explicit transactions

Validation

• cargo check --locked
• cargo test --locked --lib
• cargo test --locked --test integration_server
• ./tools/ci/run_integration_tests.sh

Related

• Implements issue feat(server): add workload governance (query timeout, cancellation, quotas) #35
• Commit: a80c9b1