fix(s3stream): prevent closed reader from restoring blocks

[BackendArchitectX]

Why

StreamReader.close() clears the current block state, but an in-flight block-index/readahead load can still complete afterward.

Because close() does not invalidate blocksEpoch, the in-flight load still sees the same epoch and can add blocks back into a reader that has already been closed.

What

Invalidate in-flight block-index loads when a StreamReader is closed.

How

Increment blocksEpoch in close() before clearing the current blocks.

Existing block-index loading already checks the captured epoch before continuing and before inserting blocks, so changing the epoch prevents work started before close from restoring stale state.

A regression test starts readahead with a delayed metadata load, closes the reader, then completes the load and verifies that blocksMap remains empty.

Verification

• ./gradlew :s3stream:test --tests com.automq.stream.s3.cache.blockcache.StreamReaderTest
• ./gradlew :s3stream:checkstyleMain :s3stream:checkstyleTest :s3stream:spotlessJavaCheck
• git diff --check

All passed locally.