chore: lint fixes part 9

[pawbana]

• testpackage (27 → 0): converted 7 test files to external test packages; exported GuessClient and GuessSessionID; merged two TestValidateProvider tests into one using NewRequestBridge; replaced
  bridge.mux.ServeHTTP with exported bridge.ServeHTTP; added //nolint:testpackage to 18 test files that heavily test unexported internals
• noctx (12 → 0): replaced http.NewRequest with http.NewRequestWithContext(t.Context(), ...) in all test files
• gocritic magic numbers (46 → 0): created internal/testutil/timeout.go with shared constants (WaitLong, WaitMedium, WaitShort, IntervalFast); replaced all magic timeout/interval values across 8 test files
• gocritic slog.Error (2 → 0): pass .Error() string instead of error value as slog.F field
• staticcheck SA9003 (1 → 0): added staticcheck to nolint directive on intentional empty recover
• confusing-naming (1 → 0): renamed openErrorResponse() → openErrBody()
• paralleltest (24 → 0): added t.Parallel() to top-level test functions and table-driven subtest loops
• tparallel (4 → 0): added t.Parallel() to missing subtests

[pawbana]

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• feat: add lint check to CI cheks #263
• chore: lint fixes part 10 #262
• chore: lint fixes part 9 #261 👈 (View in Graphite)
• chore: lint fixes part 8 #260
• chore: lint fixes part 7 #259
• chore: lint fixes part 6 #258
• chore: lint fixes part 5 #257
• chore: lint fixes part 4 #256
• chore: lint fixes part 3 #255
• chore: lint fixes part 2 #254
• chore: lint fixes part 1 (automatic fixes) #253
• feat: extend golang lint check #252
• main

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