[Improve] clean PulsarMetadataReader

[nlu90]

(If this PR fixes a github issue, please add Fixes #<xyz>.)

Fixes #

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Master Issue: #113

Motivation

Prepare the code and classes for solving #113

Modifications

1. Rename the PulsarMetadataReader class to PulsarHelper
2. Rename PulsarMetadataReader instances to pulsarHelper accordingly
3. Update several internal methods to private scope

Verifying this change

• Make sure that the change passes the CI checks.

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This change is a trivial rework / code cleanup without any test coverage.

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This change is already covered by existing tests, such as (please describe tests).

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This change added tests and can be verified as follows:

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• Added integration tests for end-to-end deployment with large payloads (10MB)
• Extended integration test for recovery after broker failure

Documentation

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