Add jittered backoff and dead-letter fields to PulsarConfig

[mauriceyap]

This extends PulsarConfig with MaxBackoffTime, BackoffRandomizationFactor, BackoffMultiplier, DeadLetterTopic, and DeadLetterMaxAttempts (validated gte=2), plus a Validate() method checking MaxBackoffTime >= BackoffTime. Note this is a pure config struct change for now - there is no consumer of these fields yet.

In a future PR, I will use cenkalti/backoff/v4 as a durect dependency to implement exponential backoff (it is currently an indirect).

[greptile-apps]

Greptile Summary

Extends Pulsar configuration for future retry and dead-letter handling.

• Adds maximum backoff, jitter randomization, multiplier, dead-letter topic, and attempt-count fields.
• Allows an omitted dead-letter attempt count while requiring configured values to be at least two.
• Adds a cross-field backoff validation method and focused unit tests.

Confidence Score: 4/5

The PR is not yet safe to merge because normal startup validation still accepts a maximum backoff shorter than the initial backoff.

The omitted dead-letter attempt count is now accepted through the omitempty constraint, but the previously reported nested-validation issue remains: production parent validators call validate.Struct and never invoke PulsarConfig.Validate, while the new tests exercise that method only directly.

Files Needing Attention: internal/common/config/pulsar.go, internal/common/config/pulsar_test.go

Important Files Changed

Filename	Overview
internal/common/config/pulsar.go	Adds retry and dead-letter configuration fields, but the new cross-field validator remains disconnected from production configuration validation.
internal/common/config/pulsar_test.go	Covers field-tag behavior and direct cross-field validation, but not validation through a production parent configuration.

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[mauriceyap]

@greptile-apps we will use the Validate() function in #5092, part of the same stack as this PR.

[greptile-apps]

Thanks for the context — that makes sense for a stacked PR workflow. The concern about Validate() being unused is resolved; happy to see it wired up in #5092.