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.

[cincuranet]

Regarding the obsoletion of PacketSize, I don't think we should put the attribute there. It is still valid setting for socket.

[cincuranet]

Suggested change

	const string connStr = "datasource=testserver;database=testdb.fdb;user=testuser;password=testpwd;blob segment size=65535";
	var cs = new ConnectionString(connStr);
	const string ConnectionString = "datasource=testserver;database=testdb.fdb;user=testuser;password=testpwd;blob segment size=65535";
	var cs = new ConnectionString(ConnectionString);

[cincuranet]

Suggested change

	const string connStr = "datasource=testserver;database=testdb.fdb;user=testuser;password=testpwd;blobsegmentsize=32000";
	var cs = new ConnectionString(connStr);
	const string ConnectionString = "datasource=testserver;database=testdb.fdb;user=testuser;password=testpwd;blobsegmentsize=32000";
	var cs = new ConnectionString(ConnectionString);

[cincuranet]

Suggested change

	const string connStr = "datasource=testserver;database=testdb.fdb;user=testuser;password=testpwd;blob segment size=100";
	var cs = new ConnectionString(connStr);
	const string ConnectionString = "datasource=testserver;database=testdb.fdb;user=testuser;password=testpwd;blob segment size=100";
	var cs = new ConnectionString(ConnectionString);

[cincuranet]

Suggested change

	const string connStr = "datasource=testserver;database=testdb.fdb;user=testuser;password=testpwd;blob segment size=70000";
	var cs = new ConnectionString(connStr);
	const string ConnectionString = "datasource=testserver;database=testdb.fdb;user=testuser;password=testpwd;blob segment size=70000";
	var cs = new ConnectionString(ConnectionString);

[cincuranet]

I think this testing is too much. The the 65535 is actually tested by test above.

Suggested change

	[Test]
	public void BlobSegmentSizeValidationMinBoundary()
	{
	const string connStr = "datasource=testserver;database=testdb.fdb;user=testuser;password=testpwd;blob segment size=512";
	var cs = new ConnectionString(connStr);
	Assert.DoesNotThrow(() => cs.Validate());
	Assert.AreEqual(512, cs.BlobSegmentSize);
	}
	[Test]
	public void BlobSegmentSizeValidationMaxBoundary()
	{
	const string connStr = "datasource=testserver;database=testdb.fdb;user=testuser;password=testpwd;blob segment size=65535";
	var cs = new ConnectionString(connStr);
	Assert.DoesNotThrow(() => cs.Validate());
	Assert.AreEqual(65535, cs.BlobSegmentSize);
	}

[cincuranet]

I guess we can remove PacketSize here, isn't it? It is not used anymore.