Fix truncated Storage uploads after retrying the final chunk

[tyler-besimple]

A failed final upload request can leave more than 256 KiB buffered after the input stream has reached EOF. uploadChunk() resets the retry size to 256 KiB, but uses EOF alone to decide whether to finalize the request and report success. The smaller retry can therefore finalize a truncated object while buffered bytes remain unsent.

Finalize only when the stream has reached EOF and the current request includes all remaining buffered bytes. Use that same condition when deciding whether to advance the buffer or complete the task.

Added two deterministic regression tests through putBytes() with mocked HTTP connections: a failed final request that stores no bytes, and one that stores 256 KiB before failing. The tests verify status recovery, upload offsets, complete byte-for-byte payloads, transferred-byte counts, and returned metadata sizes.

For the 1,310,843-byte test payload, the original code reports success after storing only 1,048,576 bytes or 1,310,720 bytes, respectively. Both tests pass with the fix.

Related to #6187. This reproduces a specific truncation mechanism; it does not establish that every report in that issue has the same cause.

Validation:

• New regression tests: both fail on unchanged production code, then pass with the fix.
• ./gradlew :firebase-storage:spotlessApply: passed.
• ./gradlew :firebase-storage:check: passed (124 debug tests and 124 release tests, no failures or skips; lint and formatting checks passed).
• git diff --check: passed.

Validation used Java 17, Android API 34, and the repository-supported subprojects.local.cfg subset for Storage and its build tooling. No public APIs or dependencies changed.

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