test(mcp): fix store tests writing fixtures outside getStateDir; sync hub-search specs with pagination API

[mojoho81]

Summary

27 tests across the 5 MCP store/route test files fail on any machine (verified identical failures on a clean checkout of main). Two independent root causes, both test-side:

Root cause A (23 tests): fixtures written outside getStateDir().
mcp-presets-store.test.ts, mcp-hub-sources-store.test.ts, mcp-tools-cache.test.ts, and -presets.test.ts set HERMES_HOME to a temp dir correctly, but then write fixture files at $HERMES_HOME/<file> — while the stores resolve their paths via getStateDir(), which appends a workspace/ segment ($HERMES_HOME/workspace/<file>). The stores never see the fixtures and bootstrap from seed data instead, so every user-file / invalid-path assertion fails.

Fix: tests now derive paths from the modules' own exported helpers (presetsFilePath(), hubSourcesFilePath(), getStateDir()) instead of hand-building them. This also un-vacuouses the two mcp-tools-cache corrupt-file tests, which previously wrote the corrupt file to a path the cache never read (they passed for the wrong reason).

Root cause B (4 tests): stale hub-search specs.
-hub-search.test.ts still expected the old 3-arg unifiedSearch signature and a limit clamp of 100; the route now clamps to 500 and forwards an offset param. Updated expectations and added offset coverage.

The only product-code change is a comment: the limit/offset docblock in hub-search.ts said 1..100 while the code clamps to 500 — corrected to match behavior.

Test results

• Before: 27 failed / 42 passed across the 5 files (same on main)
• After: 70/70 pass
• Full vitest run: no new failures introduced; eslint and tsc output identical to baseline

Notes

Test-only change (plus the one docblock line). No behavior changes. Should make these suites pass for every contributor regardless of local Hermes state or platform.