feat(logs): add generic eviction framework for pattern management

[DDuongNguyen]

What does this PR do?

Introduces a generic eviction framework (pkg/logs/patterns/eviction/) that manages bounded-size collections with configurable eviction policies:

• EvictionManager: Generic container that tracks items, enforces capacity limits, and evicts based on pluggable scoring
• Eviction scoring (score.go): Frequency × recency scoring with configurable decay, plus threshold-based eviction triggers
• Types: Evictable interface and EvictionConfig for downstream consumers (clustering, tags) to plug into

This is PR 3/6 in a stack. Depends on PR 2 (#49216). The clustering (PR 4) and tag management layers both use this framework.

Motivation

Pattern clustering and tag management both need bounded collections with LRU/LFU-style eviction. Rather than duplicating eviction logic, this PR extracts it into a reusable framework with well-tested scoring mechanics.

Describe how you validated your changes

• Comprehensive unit tests for eviction manager lifecycle, capacity enforcement, and scoring
• Score calculation tests covering decay, frequency weighting, and edge cases
• All tests pass: go test ./pkg/logs/patterns/eviction/...

How to Review this PR

1. types.go — the Evictable interface and EvictionConfig
2. eviction_manager.go — the core generic manager
3. score.go — the scoring algorithm
4. Tests for each file verify the contract

Additional Notes

This is a self-contained package with no external dependencies beyond the standard library. ~1,350 lines including tests.

[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.
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• 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
• feat(logs): add pattern clustering, tag management, and JSON processor #49218
• feat(logs): add generic eviction framework for pattern management #49217 👈 (View in Graphite)
• 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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