fix: add batchIdx to messageID.String() when batched message

[adrianiacobghiula]

Motivation

MessageID contains a batchIdx that is being skipped on String(), adding the same functionality as in java - BatchMessageIdImpl overrides the toString() to add the batchId

Modifications

add batchIdx if > -1

Verifying this change

• Make sure that the change passes the CI checks.

  • Extended unit test
  • check https://github.com/adrianiacobghiula/pulsar-client-go/actions/runs/25611371370

Does this pull request potentially affect one of the following parts:

If yes was chosen, please highlight the changes

• Dependencies (does it add or upgrade a dependency): (no)
• The public API: (no)
• The schema: (no)
• The default values of configurations: (no)
• The wire protocol: (no)

Documentation

• Does this pull request introduce a new feature? (no)

[adrianiacobghiula]

@nodece I fixed the broken integration test -> you can check first the execution on my repo https://github.com/adrianiacobghiula/pulsar-client-go/actions/runs/25611371370

[nodece]

@adrianiacobghiula Good work