feat(cache): add the arena-backed radix tree cache implementation

[vedantdas-source]

Description

This PR introduces initial structure of the arenaRadix cache.

To reduce memory footprint and garbage collection overhead, this implementation moves away from traditional pointer-based nodes. Instead, it utilizes a Left-Child Right-Sibling (LCRS) representation backed by a slice-based arena allocator ([]arenaRadixNode) and 32-bit indices.

Changes made

• added arenaRadixNode and arenaRadix core structs.
• Internal arena memory management via a custom freelist (allocateNode, freeNode), entirely eliminating heap allocations during tree node creation.
• Helper utilities for deterministic key hashing and full path reconstruction.

Link to the issue in case of a bug fix.

NA

Testing details

1. Manual - NA
2. Unit tests - Added a comprehensive suite in arena_radix_test.go to explicitly verify the internal tree mechanics and lru operations
3. Integration tests - NA

Any backward incompatible change? If so, please explain.

None. This PR introduces new internal logic alongside the existing implementations and does not yet mutate any public package APIs or interfaces.

[gemini-code-assist]

Code Review

This pull request introduces a custom radix tree implementation (arenaRadix) using a Left-Child Right-Sibling (LCRS) representation and 32-bit indices to minimize slice allocations and pointer overhead, along with comprehensive unit tests. The feedback is minor, pointing out a typo ('capcacity') in a panic message within the node allocation logic.

[codecov]

Codecov Report

❌ Patch coverage is 91.77489% with 19 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (radix-trie-cache@dce6bfb). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
internal/cache/lru/arena_radix.go	91.77%	11 Missing and 8 partials ⚠️

Additional details and impacted files

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##             radix-trie-cache    #4878   +/-   ##
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  Hits                        ?    18042           
  Misses                      ?     2788           
  Partials                    ?      674           

Flag	Coverage Δ
unittests	83.90% <91.77%> (?)

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[kislaykishore]

Please add the LRU implementation too in the same PR and ensure that the tests that pass with other LRU implementations pass with this one as well.

[vedantdas-source]

Please add the LRU implementation too in the same PR and ensure that the tests that pass with other LRU implementations pass with this one as well.

Done. Added the functionality of the LRU operations as well as tests that the existing lru cache are tested against.