feat(logs): wire stateful encoding into log agent pipeline

[DDuongNguyen]

What does this PR do?

Wires the stateful encoding feature into the log agent's pipeline and status reporting:

• Pipeline integration (pkg/logs/pipeline/): pipeline.go and provider.go updated to instantiate the gRPC sender with pattern encoding when use_grpc is enabled
• Agent implementation (comp/logs/agent/agentimpl/): agent.go and agent_test.go updated for the new sender construction path
• Status reporting (pkg/logs/status/, status templates): Adds stateful encoding state (active patterns, cluster count, cache stats) to agent status output
• Message type (pkg/logs/message/message.go): Extends the Message struct to carry pattern metadata through the pipeline

This is PR 6/6 in a stack. The final integration layer that connects all previous PRs into the running agent. Depends on PR 5 (#49219).

Motivation

All the building blocks (tokens, tokenizer, eviction, clustering, sender) are in place — this PR connects them to the actual log agent lifecycle so the feature activates when configured.

Describe how you validated your changes

• Existing agent tests updated and passing
• Status output includes new stateful encoding section
• End-to-end manual testing with use_grpc: true configuration

How to Review this PR

1. pkg/logs/pipeline/provider.go — the main wiring point where the gRPC sender is selected
2. comp/logs/agent/agentimpl/agent.go — agent lifecycle changes
3. pkg/logs/status/builder.go — new status fields
4. Status templates — verify the display format
5. This is intentionally the smallest PR in the stack (~40 net lines)

Additional Notes

This is the thinnest PR in the stack by design — it only wires existing components together. If any upstream PR needs changes, this one will likely need a trivial rebase.

[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 👈 (View in Graphite)
• feat(logs): add pattern extraction to gRPC stateful sender #49219
• 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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