fix(mypy): resolve OpenTelemetry typing issues in telemetry.py (#3931)

## Summary

Fix all 11 mypy type checking errors in `telemetry.py` without using any
type suppressions.

**Changes:**
- Add type aliases for OpenTelemetry attribute types (`AttributeValue`,
`Attributes`)
- Create `_clean_attributes()` helper to filter None values from
attribute dicts
- Use `cast()` for TracerProvider methods (`add_span_processor`,
`force_flush`)
- Use `cast()` for metric creation methods returning from global storage
- Fix variable reuse by renaming `span` to `end_span` in SpanEndPayload
branch
- Add None check for `parent_span` before `set_span_in_context`

**Errors Fixed:**
- TracerProvider attribute access: 2 errors
- Counter/UpDownCounter/ObservableGauge return types: 3 errors
- Attribute dict type mismatches: 4 errors
- Span assignment type conflicts: 2 errors

**Testing:**
```bash
uv run mypy src/llama_stack/core/telemetry/telemetry.py
# Success: no issues found
```

**Part of:** Mypy suppression removal plan (Phase 2a/4)

**Stack:**
- [Phase 1] Add type stubs (#3930)
- [Phase 2a] Fix OpenTelemetry types (this PR)
- [Phase 2b+] Fix remaining errors (upcoming)
- [Phase 3] Remove inline suppressions (upcoming)
- [Phase 4] Un-exclude files from mypy (upcoming)

Author: ashwinb