Add read-only KV cache observability helper module for step-level tracing.
Provides utilities to extract per-request and per-step KV cache metrics
using only existing exposed interfaces.

Key additions:
- vllm/v1/core/kv_cache_observability.py: PerRequestKVMetrics and
  StepKVSummary dataclasses with query functions
- tests/v1/core/test_kv_cache_observability.py: 18 unit tests with
  minimal fakes (17 fake-based + 1 smoke test)

Design principles:
- Read-only access to existing KV cache state
- Defensive programming (never raises exceptions)
- Aggregates across all KV cache groups (multi-group support)
- Guaranteed GPU metrics + optional best-effort fields
- No changes to KV cache behavior, scheduler, or Request fields
- No new APIs or expensive scans
- Python 3.9+ compatible (uses __future__ annotations)

Implementation details:
- Aggregates blocks across all single_type_managers (not just [0])
- Defensive clamping for blocks_total (prevents negative values)
- Conservative usage_ratio fallback (0.0 when unmeasurable)
- Tests use minimal fakes (no scheduler coupling)
- Fast, deterministic tests (2.48s, no heuristics)

All 18 tests passing. Zero impact on existing functionality.

Part of Step-Level Tracing implementation (PR #4 of 5).
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>