Add task-level cache max age support

[pvditt]

Summary

• add Cache.max_age as a task-level cache policy, accepting a nonnegative timedelta or integer number of seconds
• serialize the setting to TaskMetadata.cache_max_age, preserving the distinction between unset and explicit zero
• round-trip and override the setting through remote task details
• validate positive, zero, unset, negative, and invalid-type behavior

None uses the platform default, zero disables age-based expiration, and a positive value limits the age of cached results that the task will reuse.

IDL dependency

This depends on TaskMetadata.cache_max_age from flyteorg/flyte#7938 (commit). The dependency pin is intentionally not changed in this PR, so CI is expected to fail against flyteidl2==2.0.44 until a new IDL version is published and the SDK pin is bumped.

Testing

Using a locally built flyteidl2==2.0.45.dev0 wheel:

• pytest -q tests/user_api/test_cache.py tests/flyte/internal/runtime/test_task_serde.py tests/flyte/remote/test_task.py tests/flyte/test_image.py — 177 passed
• Ruff passed for all changed source and test files
• git diff --check passed

Dogfood validation

Validated against dogfood/flytesnacks/development and the leaseworker implementation in unionai/cloud#18043. The expiring task template contained cacheMaxAge: 60s; the normal-cache task omitted the field.

Scenario	Run	Cache status	Execution marker
Initial 60s cache	expiring-3	CACHE_MISS	04:46:48.396163Z
Immediate repeat	expiring-4	CACHE_HIT	Same marker
Repeat after 163s	expiring-5	CACHE_MISS	04:49:31.529448Z
Immediate repeat of refreshed result	expiring-6	CACHE_HIT	Same refreshed marker
Normal cache initial	normal-1	CACHE_MISS	04:47:39.246597Z
Normal cache immediate repeat	normal-2	CACHE_HIT	Same marker
Normal cache after the wait	normal-3	CACHE_HIT	Still the original marker

Dogfood workers were still using an older SDK without the task-side constructor argument, so the probe used this SDK build for registration and an import-time compatibility guard for the older task runtime. Cache lookup and expiration occur in leaseworker before task execution.

[wild-endeavor]

don't we need to bump something in pyproject? or did someone else already do that? what's the min idl version needed for this?

[wild-endeavor]

oh you were waiting on a new version - looks like someone published https://pypi.org/project/flyteidl2/2.0.45/