fix(firestore): bubble up errors in BulkWriter  (#14481)

This PR addresses an issue where the Firestore `BulkWriter` could
silently fail to persist documents without notifying the caller. This
behavior was primarily observed when the `BulkWriter`'s context was
canceled or when internal limits were reached.

#### Root Causes
1. **Dropped Batches on Cancellation:** In the background `send`
function, if the context was canceled, the current batch of jobs was
dropped immediately without notifying the individual job result channels
.
2. **Ignored Queuing Errors:** The `write` function and the retry logic
ignored errors returned by the underlying bundler when attempting to
queue a write .
3. **Ignored Rate Limit Errors:** Context errors occurring during the
rate limiter's wait period were not checked or propagated .

#### Changes
- Updated the internal `write` method and its public callers (`Create`,
`Delete`, `Set`, and `Update`) to return errors from the rate limiter
and the bundler.
- Modified the `send` function to iterate through and notify all jobs in
a batch with a context error if the transmission is aborted due to
cancellation.
- Added error checking to the retry logic within `send` to ensure
failures to re-queue a job are surfaced.

These changes ensure that any failure to queue or send a document is
explicitly reported through the `BulkWriterJob` results or as an
immediate return value, preventing data loss.

#11422.

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Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>