feat: add GetOrTrySetFunc to support fallible value loading

[AdamMagued]

Summary

Adds GetOrTrySetFunc to Cache to support fallible value computation and loading without caching transient errors or partial failures.

Problem

Currently, GetOrSetFunc executes a value factory func() V and unconditionally inserts the result into the cache. When loading values that may fail (e.g., transient network or database errors), callers must either catch the error outside and manually delete the cached entry or avoid GetOrSetFunc, which can lead to race conditions or caching invalid data.

Solution

Introduces GetOrTrySetFunc(key K, fn func() (V, bool), opts ...Option[K, V]) (*Item[K, V], bool):

• If the item is already present in the cache and not expired, it is returned with retrieved=true without executing fn.
• If the item is missing or expired, fn is executed under the cache mutex.
• If fn returns cancel=true, the operation is aborted: no item is inserted or overwritten in the cache, and (nil, false) is returned.
• If fn returns cancel=false, the item is created, stored with the configured options (such as TTL), and returned with (item, false).
• Subsequent attempts for a key that previously cancelled will re-execute the loader function.

Verification

• Added Test_Cache_GetOrTrySetFunc covering successful set, hit without execution, cancellation on cache miss, loader re-execution after cancellation, and expired item handling.
• Verified test suite passes: go test -v ./... and go test -race ./....

Fixes #188