GeezerVison

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 PacketSize default, so default behavior is unchanged).
• Exposed via FbConnectionStringBuilder.BlobSegmentSize and FbConnection.BlobSegmentSize.

PacketSize is unchanged

• PacketSize keeps its meaning: it sizes the socket send/receive buffers of the managed client. It is just no longer (ab)used as the blob segment size.
• DatabaseBase.PacketSize was unused once blobs read BlobSegmentSize, so it was removed (together with the packetSize parameter of FesDatabase). GdsConnection.PacketSize remains, as the only consumer.

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  (replaces DatabaseBase.PacketSize)
  → BlobBase._segmentSize  (was db.PacketSize, now db.BlobSegmentSize)

Tests

• ConnectionStringTests: 6 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.