tests: converge the three same-interface createMockFileSystem() helpers (divergent defaults)

[debugmcpdev]

Follow-up from #739 / #741, which made tests/test-utils/helpers/test-utils.ts's createMockFileSystem() honest (no as any, all 16 IFileSystem members) but deliberately did not converge the four definitions that exist. They differ in ways their consumers rely on, so this is a ticket rather than a ride-along:

Definition	Type	Defaults	Consumers
tests/test-utils/helpers/test-utils.ts	@debugmcp/shared IFileSystem, no cast	pre-configured: pathExists→true, existsSync→true, ensureDir→undefined, readTail/readFile→'', stat→{isDirectory:()=>false,isFile:()=>true}, …	5 SessionManager-layer suites (proxy-failure-diagnostics, session-manager-attach-modes, -launch-gate, session-manager-test-utils, session-manager-operations-coverage); three carry a comment relying on pathExists→true
tests/test-utils/helpers/test-dependencies.ts:63	same interface, no cast	none — every member a bare vi.fn()	proxy-manager-factory.test.ts, proxy-manager-message-handling.test.ts, and createMockDependencies()
tests/test-utils/helpers/adapter-dependencies.ts:33	same interface, (overrides: Partial<IFileSystem> = {})	exists/pathExists/existsSync→**false**, spreads overrides	tests/unit/test-utils/test-proxy-manager.ts (aliased createTypedMockFileSystem)
tests/test-utils/mocks/dap-proxy-doubles.ts	the worker-local four-member IFileSystem in src/proxy/dap-proxy-interfaces.ts	—	proxy-worker + go-initialized-fallback tests

The fourth is a different interface on purpose (a wide double would hide a member the worker starts calling) and should stay. The first three are the same interface with three default policies: "everything exists", "nothing answers", "nothing exists + overrides". Consolidating onto any one of them silently changes what the other two's consumers see — test-dependencies.ts's bare version would break the three suites that rely on pathExists→true; adapter-dependencies.ts's false default would too.

Suggested shape: one createMockFileSystem(overrides?: Partial<IFileSystem>) with the "everything exists" defaults (the majority case) in helpers/test-dependencies.ts, createMockDependencies() using it, and the two other files re-exporting or deleted; the test-proxy-manager.ts caller passes { pathExists: vi.fn().mockResolvedValue(false), … } explicitly where it needs the negative. Gate: npx tsc -p tsconfig.spec.json silent and every listed consumer green in isolation.

Same class as #695 / #739 (drifted or duplicated test doubles).