docs: cover binding tests for expensive commands

[maliberty]

Extends docs/agents/testing-strategy.md to cover the case the Tier-2 recipe doesn't: commands whose execution is expensive (global_route, global_placement, clock_tree_synthesis, detailed_route), where "run it once on a trivial design" is the wrong cost trade-off.

• States the preferred policy: validate arguments in C++, not in the binding — a .tcl check only guards the Tcl path, while a check behind the C++ entry point (via utl::Validator) covers Tcl, Python, and direct C++ callers from one place.
• Explains how to prove the translation contract without executing the engine: rename the thin execute free function to a spy so argument handling runs but the algorithm doesn't.
• Documents the three real binding shapes (setters + separate execute; args forwarded to C++; fused configure+execute) and what to assert in each, with detailed_route_cmd called out as the fused outlier worth refactoring.
• Cross-links from the Tier-2 section and adds a line to the decision tree.

[gemini-code-assist]

Code Review

This pull request updates the testing strategy documentation to outline a strategy for testing expensive commands. It recommends validating arguments in C++ and separating the configuration/marshalling step from the execution step so that binding tests can spy on the execution without running the expensive algorithm. The review feedback focuses on improving the documentation's accuracy and consistency, specifically by standardizing the spelling of 'marshaling', correcting Python terminology (using 'function' instead of 'proc'), and clarifying how Python tests can mock or reassign functions similarly to Tcl's 'rename' command.

[chatgpt-codex-connector]

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