perf(container): use iterator lookup instead of exception handling in Map/Dict

[junrushao]

Summary

• Replace try { n->at(k); } catch (...) with n->find(k) + iterator check in ffi.MapGetItemOrMissing and ffi.DictGetItemOrMissing
• Avoids the overhead of throwing and catching exceptions on every missing-key lookup
• No behavioral change: both paths return GetMissingObject() for absent keys

Closes #547

Test plan

• uv run pytest -vvs tests/python -- 2088 passed, 38 skipped, 3 xfailed (no new failures)
• Pre-commit hooks pass (clang-format, ASF header, file types, etc.)

[gemini-code-assist]

Code Review

This pull request updates the ffi.MapGetItemOrMissing and ffi.DictGetItemOrMissing functions in src/ffi/container.cc to use iterator-based lookups with find instead of try-catch blocks around at. This modification improves efficiency by avoiding the overhead associated with exception handling for missing keys. I have no feedback to provide.

[gemini-code-assist]

Code Review

This pull request refactors the ffi.MapGetItemOrMissing and ffi.DictGetItemOrMissing functions in src/ffi/container.cc to use the find method instead of at within a try-catch block. This change improves efficiency by avoiding the overhead of exception handling when a key is missing. I have no feedback to provide as there were no review comments.