feat(logs): add pattern extraction to gRPC stateful sender

[DDuongNguyen]

What does this PR do?

Integrates pattern extraction into the gRPC stateful sender (built in #48879 by @TheSafo), adding:

• MessageTranslator with pattern caching (sender.go): Attaches pattern signatures and cluster IDs to outgoing log messages, caching state to avoid redundant computation
• Batch strategy (batch_strategy.go): Pattern-aware batching that considers signature similarity when grouping messages for transmission
• Stream worker updates (stream_worker.go): Wires the MessageTranslator into the existing gRPC streaming pipeline
• Inflight tracking (inflight.go): Tracks in-flight pattern state for acknowledgment handling
• State telemetry (state_telemetry.go): Telemetry counters for pattern cache hits/misses, encoding decisions
• Mock helpers (mock_encoder.go, mock_state.go, mock_state_cache_test.go): Test infrastructure for the pattern-aware sender

This is PR 5/6 in a stack. Depends on PR 4 (#49218) for the clustering and tag modules. This PR modifies the gRPC sender from #48879 — it represents only the delta of pattern extraction additions on top of Jake's transport layer.

Motivation

The gRPC sender needs to attach pattern metadata to log messages so the backend can perform stateful encoding (deduplication, compression). This PR adds that translation layer without changing the underlying transport mechanics.

Describe how you validated your changes

• Unit tests for MessageTranslator caching behavior (mock_state_cache_test.go)
• Batch strategy tests for pattern-aware grouping
• Stream worker tests updated for the new encoding flow
• Metrics module tests for telemetry counters

How to Review this PR

1. sender.go — the MessageTranslator and its pattern cache
2. batch_strategy.go — how pattern similarity affects batching
3. stream_worker.go — diff is small, focus on the MessageTranslator integration points
4. state_telemetry.go — new telemetry counters
5. Tests validate each component independently

Additional Notes

This PR modifies files that already exist in #48879's branch. The diff shows only incremental additions (pattern extraction logic), not the full sender implementation.

[DDuongNguyen]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• utf8 #49898 : 3 dependent PRs (#49961 , #50036 , #50047 )
• fix(logs): staging fidelity fixes — tabs, newlines, Unicode positions, UTF-8 safety #49810
• better json #49672
• local proto plus servic #49628
• feat(logs): periodically remove unused entry from dict #49494
• feat(logs): max template size limit + json_as_raw raw bypass #49552
• fix(logs): send delete on tag eviction #49493
• fix(logs): rebuild libpatterns.so targeting glibc 2.30 (was 2.34) #49446
• feat(logs): send JSON via dict-encoded sets of keys #49404
• feat(logs): add dual-send shadow gRPC pipeline #49327
• feat(logs): wire stateful encoding into log agent pipeline #49220
• feat(logs): add pattern extraction to gRPC stateful sender #49219 👈 (View in Graphite)
• feat(logs): add pattern clustering, tag management, and JSON processor #49218
• feat(logs): add generic eviction framework for pattern management #49217
• feat(logs): add Rust FFI tokenizer bridge with FlatBuffers #49216
• feat(logs): add stateful encoding module plumbing and token types #49215
• [Logs Stateful Encoding] transport layer initial implementation #48879
• logs-stateful-main

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