fix(processor): replace panic with graceful error in activePartitions

[sriman0707]

Description

Replaces a panic(err) call in processor.activePartitions() with graceful error handling.

Previously, when the database connection was temporarily lost, activePartitions() would panic. This panic terminated the runner's pinger goroutine, silently crashing the active event-processing loop.

With this change, the handler logs the connection error, records a metric, and returns an empty partition list. This allows the pinger loop to gracefully retry fetching active partitions on the next tick once the database recovers.

Linear Ticket

N/A (External Open Source Contribution)

Security

• The code changed/added as part of this pull request won't create any security issues with how the software is being used.

[contributor-support]

Thank you @sriman0707 for contributing this PR.
Please sign the Contributor License Agreement (CLA) before merging.

[github-actions]

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[sriman0707]

Hi maintainers,

Just a friendly follow-up on this PR.

I have completed the CLA requirements and kept the branch up to date with the latest master. The change replaces a panic with graceful error handling, preventing the processor loop from terminating when the database connection is temporarily unavailable.

I'd appreciate it if someone could take a look when you have time. If there are any changes, tests, or improvements you'd like me to make, I'm happy to update the PR.

Thanks for your time and for maintaining RudderStack!

[gitcommitshow]

Thanks for the PR @sriman0707
Can you please help me understand few more things

1. Was any AI was used in creating this PR?
2. How did you discover the problem you are fixing in this PR?

[sriman0707]

Thanks for asking!

1. Was AI used in creating this PR?
   Yes. I used Antigravity with my own custom security skill file to assist with the source-code analysis. I first went through SAST and then used the workflow to investigate the relevant source-code paths and understand the issue. The analysis and changes were reviewed and validated by me before creating the PR.

2. How did you discover the problem you are fixing in this PR?
   I initially identified the area through SAST analysis and then investigated the relevant source code to understand how "activePartitions" was being handled. During the source-code review, I found that the current behavior could result in a panic under certain conditions instead of handling the situation gracefully. I reproduced/validated the behavior, worked on the fix, and then created this PR to replace the panic with graceful error handling.