refactor: move empty extent trimming to ExtentList

[shiyudesu]

Description

Move the empty extent filtering logic from the delta proxy cache implementation into timeseries.ExtentList.

This change:

• adds ExtentList.TrimEmptyExtents();
• updates fetchExtents to use the new method;
• removes the standalone trimEmptyExtents function from deltaproxycache.go;
• adds table-driven unit tests covering interspersed empty extents, all-empty lists, empty lists, and lists without empty extents.

This keeps the existing behavior while making ExtentList responsible for managing its own representation.

Closes #1126

Type of Change

• • Bug fix
• • New feature
• • Optimization
• • Test coverage
• • Documentation
• • Infrastructure

Testing

• make build
• make lint
• make test
• make data-race-test

All checks pass. The project test suite reports 91.02% statement coverage, and TrimEmptyExtents has 100% statement coverage.

AI Disclosure

• • This contribution DOES NOT include AI-generated changes
• • This contribution DOES include AI-generated changes, and I have reviewed the relevant contributing guidelines.

[jranson]

integration test failures appear unrelated to this change, so we'll resolve them separately

--- FAIL: TestPCFNeverCollapsesPost
--- FAIL: TestPCFRefusesIneligibleResponses