test(server): add unit coverage for the store_budget LRU key ordering and JSON length helpers

## Summary

`src/server/store_budget.rs`, the shared helper module for the bounded in-memory server stores, has no tests, while every sibling helper module under `src/server/` carries a `_tests.rs`. Add direct unit coverage for its two pure helpers.

## Background

The module exposes `serialized_json_len_saturating` (a saturating JSON byte counter, `src/server/store_budget.rs:28`) and the `LruKey` `Ord` implementation whose tiebreak chain (last_accessed, then sequence, then id) determines eviction order. Both `responses_store.rs` and `conversation_store.rs` depend on these to stay correctly bounded, so the ordering is worth pinning. A grep for either symbol finds no test anywhere.

## Proposed Solution

Add a `store_budget_tests.rs` wired via `#[cfg(test)]` in `src/server/mod.rs`, matching the existing sibling pattern. Cover the JSON length saturation path and a sort of hand-built `LruKey` values that exercises each tiebreak level. No MLX or model dependency is involved.

## Implementation Notes

- `src/server/store_budget.rs:28` defines `serialized_json_len_saturating`, backed by the `SaturatingByteCounter` writer that returns `usize::MAX` on serialization error.
- The `LruKey` `Ord` impl (last_accessed then sequence then id) is the eviction contract used by the bounded stores.
- Follow the `src/server/*_tests.rs` convention already present for the sibling helpers.

## Acceptance Criteria

- [ ] Both helpers have direct unit coverage, including the `Ord` tiebreak chain
- [ ] Tests follow the `src/server/*_tests.rs` convention

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## Original Suggestion

### Title: test(server): add unit tests for the store_budget helpers

`src/server/store_budget.rs` — the shared helper module for the bounded in-memory server stores — has zero tests, while every sibling helper module in `src/server/` carries a `_tests.rs`.

## Evidence

- `src/server/store_budget.rs:28-78` — `serialized_json_len_saturating` and the `LruKey` `Ord` impl (last_accessed → sequence → id tiebreak) are pure and allocation-free; a grep for either symbol finds no test anywhere
- The eviction order encoded in that `Ord` impl is exactly what keeps the response/conversation stores bounded correctly — worth pinning

## Suggested fix

Add a `store_budget_tests.rs` wired via `#[cfg(test)]` in `src/server/mod.rs`, matching the existing sibling pattern: a few asserts on the JSON length saturation and a sort of hand-built `LruKey`s covering each tiebreak level. No MLX or model dependency involved.

## Acceptance criteria

- [ ] Both helpers have direct unit coverage, including the `Ord` tiebreak chain
- [ ] Tests follow the `src/server/*_tests.rs` convention

Start with src/server/store_budget.rs and the existing src/server/*_tests.rs sibling pattern, then inspect src/server/mod.rs for test-module wiring. Add direct tests for serialized_json_len_saturating and hand-built LruKey values covering the last_accessed, sequence, and id tiebreaks. Done means both helpers have coverage and the tests follow the established convention.

Written by the indexing model from the issue text.