pulsar: make max-message-bytes configurable

Is your feature request related to a problem?

The Pulsar sink hardcodes the maximum message size to 10MB (DefaultMaxMessageBytes) in downstreamadapter/sink/pulsar/helper.go. Users whose Pulsar brokers are configured with a different maxMessageSize have no way to align TiCDC's encoder limit with their broker configuration, which can cause silent message truncation or send failures.

Describe the feature you'd like

Add a max-message-bytes URI parameter to the Pulsar sink, matching the existing Kafka sink behavior:

pulsar://host:6650/topic?protocol=canal-json&max-message-bytes=5242880

Implementation:

• Add MaxMessageBytes *int field to PulsarConfig in pkg/config/sink.go.
• Parse the parameter from the sink URI in pkg/sink/pulsar/config.go, defaulting to 10MB when omitted.
• Pass the configured value to GetEncoderConfig in downstreamadapter/sink/pulsar/helper.go.

Describe alternatives you've considered

Users currently have no workaround other than patching the source code or ensuring all messages stay under 10MB. The Kafka sink already supports this parameter, so the pattern is well established in the codebase.

Teachability, Documentation, Adoption, Migration Strategy

The Pulsar sink URI parameter documentation should list max-message-bytes with its default value (10485760, i.e. 10MB). No migration is needed for existing changefeeds since the default is unchanged.

Start by comparing the existing Kafka sink handling for max-message-bytes, then inspect pkg/config/sink.go, pkg/sink/pulsar/config.go, and downstreamadapter/sink/pulsar/helper.go. The work is complete when the Pulsar URI accepts the parameter, defaults to 10485760, passes it to the encoder configuration, and the sink documentation lists it.