Add observed budgets for native agent runs - #5555

Summary

This pull request adds optional budgets for direct native agent runs and Session submissions. Applications can limit admitted root requests, reported tokens, and estimated cost in integer micro-US dollars.

The SDK records reported usage before deciding whether to admit another request. A denial happens before the step hook and provider request. If a measurement required by the budget is missing, the default policy stops the next request. Applications can opt to continue with the measurements they have. An admitted response can use more than the limit and still return a valid final result.

Live manual replies, retained steering, and output repair share the current budget. New root operations start with a fresh budget. Active budgets prevent portable checkpoint capture and preserve the live reply.

Test plan

• The repository verification wrapper passed format, lint, type checking, and tests on Python 3.14. The parallel suite passed 4,937 tests with 176 skips. The serial suite passed 41 tests.
• Fresh wheel installs on Python 3.10, 3.13, and 3.14 passed repository qualification, 41 budget cases each, public annotation checks, and the live checkpoint probe.
• The generated API contract passed its standard check. Independent runtime and public API reviewers verified the feature implementation.
• The Python 3.10 MCP traceback tests passed in hosted Linux CI before the rebase. This branch now uses the shared correction from the target branch.
• The Windows child-stop test now waits for native turn handles before taking its snapshot. The corrected test passed five times on the qualified Python 3.10 wheel. Hosted CI is running on the rebased head.
• An earlier editable test run crashed while another install wrote to the same native addon. The cause remains unknown. The final verification completed with the same addon hash and file metadata before and after the run.

Checks

• Added native and public API regression tests
• Ran the applicable repository verification wrapper
• Completed independent runtime and public API review
• All required checks pass