Support higher maximum blob segment size (#1197)

[fdcastel]

Fixes #1197.

Summary

This PR implements support for blob segment sizes up to 65535 bytes (64 KiB − 1), as supported by Firebird since version 2.1 (and fully reliable in Firebird 3.0+).

Changes

New BlobSegmentSize connection string property

• BlobSegmentSize is a new connection string key (synonym: blobsegmentsize) that controls the segment size used when reading/writing blobs.
• Valid range: 512–65535 bytes. Default: 8192 bytes (same as the old PacketSize default).
• Exposed via FbConnectionStringBuilder.BlobSegmentSize and FbConnection.BlobSegmentSize.

PacketSize deprecation

• FbConnectionStringBuilder.PacketSize and FbConnection.PacketSize are marked [Obsolete] with a message pointing to BlobSegmentSize.
• PacketSize still works—it continues to control socket buffer sizing—but it is no longer used as the blob segment size.

Raised wire protocol limits

• GdsBlob.GetSegment (all 4 variants): cap changed from short.MaxValue - 12 (32755) to ushort.MaxValue (65535).
• XdrReaderWriter.WriteBlobBuffer: guard changed from short.MaxValue to ushort.MaxValue.
• IFbClient.isc_get_segment / isc_put_segment: parameter types changed from short / ref short to ushort / ref ushort, matching the Firebird C API's actual unsigned short types. FesBlob updated accordingly.

Stack plumbing

BlobSegmentSize flows through the full stack:

ConnectionString.BlobSegmentSize
  → GdsConnection.BlobSegmentSize / FesDatabase(blobSegmentSize)
  → DatabaseBase.BlobSegmentSize
  → BlobBase._segmentSize  (was db.PacketSize, now db.BlobSegmentSize)

Tests

• ConnectionStringTests: 7 new tests (default, parsing, synonym, validation boundaries).
• FbConnectionStringBuilderTests: 3 new tests (default, setter, getter).
• FbBlobTests: 2 new integration tests exercising blob read/write with BlobSegmentSize = 65535.