feat: add reference count to subscription table

[micpapal]

Description

Previously, when multiple consumers within a session subscribed to the same name, each subscription would send a separate subscribe message to the data plane. Similarly, each unsubscribe would send an unsubscribe message. This lead to issues where a subscription was removed from the data plane while other consumers still needed it.

This PR implements reference counting in the subscription table to prevent this problem. It also removes the session route cache in the session layer as the ref count is now directly implemented in the subscription table.

Ref: #1138

Type of Change

• Bugfix
• New Feature
• Breaking Change
• Refactor
• Documentation
• Other (please describe)

Checklist

• I have read the contributing guidelines
• Existing issues have been referenced (where applicable)
• I have verified this change is not present in other open pull requests
• Functionality is documented
• All code style checks pass
• New code contribution is covered by automated tests
• All new and existing tests pass

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf CI / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Jan 27, 2026, 10:47 AM

[codecov]

Codecov Report

❌ Patch coverage is 92.94118% with 18 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...ane/core/datapath/src/tables/subscription_table.rs	92.07%	18 Missing ⚠️

📢 Thoughts on this report? Let us know!