Firebird Proxy does not enforce a maximum buffered BLOB write size (isc_blobtoobig)

Split off from #39052 at makssent's suggestion (https://github.com/apache/shardingsphere/issues/39052#issuecomment-5581713843), since this needs a program-logic change rather than just wiring an existing signal to an error response.

## Bug Report

### Expected behavior

Once a client-uploaded BLOB (via op_put_segment / op_batch_segments) exceeds some maximum buffered size, further segments for that BLOB should keep failing with isc_blobtoobig instead of being silently accepted.

### Actual behavior

`FirebirdBlobWrite` buffers an in-flight BLOB write into an unbounded `ByteArrayOutputStream` (`proxy/frontend/dialect/firebird/.../command/query/blob/cache/FirebirdBlobWrite.java`). There is no size tracking or limit anywhere in the BLOB write path, so a client can buffer an arbitrarily large BLOB into proxy heap memory before it is ever routed to a backend.

### Why this needs its own issue

Unlike the other four sub-cases in #39052, this one isn't "surface an error the cache layer already detects" — no such signal exists yet. It needs:
- A decision on where the limit comes from (a hardcoded constant vs. a configurable proxy property), since Firebird itself has no fixed protocol-level BLOB size ceiling — this would be a ShardingSphere-proxy-specific safety guard against unbounded heap buffering.
- Once decided, tracking buffered size against that limit in `FirebirdBlobWrite`/`FirebirdBlobWriteCache`, and returning isc_blobtoobig (already scaffolded via the #38927 error-handling pattern) from `FirebirdPutBlobSegmentCommandExecutor`/`FirebirdBatchBlobSegmentsCommandExecutor` once exceeded, with segments received after the limit continuing to fail rather than being accepted again.

### Related

- #39052 (parent issue, now scoped to the other 4 sub-cases)
- #38927 (established the Firebird dialect error-handling pattern this would extend)

Start with FirebirdBlobWrite and FirebirdBlobWriteCache to understand how buffered segments are stored, then inspect FirebirdPutBlobSegmentCommandExecutor and FirebirdBatchBlobSegmentsCommandExecutor alongside the #38927 error-handling pattern. Decide how the proxy limit is supplied, enforce it for each BLOB, and ensure segments received after the limit return isc_blobtoobig; verify the affected command paths and existing tests.

Written by the indexing model from the issue text.