[Mola-maker]

A cached full-object entry carries no precondition, so serving it to a conditional read would never validate the condition against the origin: after a fill, a later read with a different if-match (or any other precondition) would return the cached bytes with Ok even when the live object no longer satisfies it.

This change makes a conditional read fall back to the origin when an entry for its key is already cached. On a cache miss the fill path is unchanged: it still validates the condition at the origin while filling, as fixed by #7718. version keeps using the cache because it is part of the cache key.

Tests:

• New test test_conditional_read_bypasses_full_object_cache: fills the cache with an unconditional read, then asserts a conditional read contacts the origin again and that later unconditional reads still hit the cache. It failed before the fix (origin open calls stayed at 1) and passes after.
• cargo test -p opendal-layer-foyer: 10 lib tests + 2 doc tests pass.
• cargo test -p opendal-core --lib: 236 passed.
• cargo clippy -p opendal-layer-foyer --all-targets -- -D warnings and cargo fmt --all -- --check clean.

Fixes #7686.

AI Usage: Harness: Muse; Model: Muse Spark; Effort: investigation and tests.