fix[next]: Catch exceptions thrown in async build pool

[edopao]

The executor call in compiled backends runs as a future when asynchronous compilation is enabled. The current code did not catch exceptions thrown by the future, so any lowering/compilation error resulted in a KeyError at runtime when calling the corresponding program. This PR addresses this issue and adds test coverage.

A side effect of this change is that unit tests that call wait_for_compilation() in a multi-thread pytest session might catch exceptions thrown by other tests, because the build thread pool is a shared resource. This PR limits the pool size to 1, so that the compile failure can be caught in the unit test where it originates from.

[Copilot]

Pull request overview

This PR fixes error surfacing for asynchronous compilation in gt4py.next compiled backends by tracking submitted compilation futures and re-raising their exceptions during gtx.wait_for_compilation(), preventing later runtime failures (e.g., KeyError on program call). It also extends integration test coverage to exercise both synchronous and asynchronous compilation modes under executor failure.

Changes:

• Track async compilation futures in a global registry and re-raise Future exceptions in wait_for_compilation().
• Register each submitted async compilation job in the new pending-futures registry.
• Extend test_wait_for_compilation to cover success/failure and synchronous/asynchronous compilation modes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/gt4py/next/otf/compiled_program.py	Adds global pending-futures tracking and surfaces async compilation exceptions via wait_for_compilation().
tests/next_tests/integration_tests/feature_tests/ffront_tests/test_compiled_program.py	Expands integration test matrix to validate exception surfacing behavior across sync/async compilation.