Add 156 unit tests for under-covered modules

[devin-ai-integration]

Summary

Adds 156 new unit tests (234 → 390 total) targeting modules that had zero or minimal test coverage.

New Hookline.SharedKernel.Tests project — previously untested:

• ResultTests — Result.Success(), Result.Failure(error), Result<T> implicit operators (T → Result<T>, Error → Result<T>), predefined Error status codes
• PagedResultTests — TotalPages ceiling division, HasNext/HasPrevious edge cases, PageRequest.SafePage/SafePageSize clamping (negative, zero, >MaxPageSize), Skip offset
• CsvTests — RFC 4180 Field() quoting (comma, quote, CR, LF triggers), Row() joining, Document() CRLF termination

Infrastructure.Tests additions:

• PasswordHasherTests — bcrypt hash format, nondeterminism, verify correct/wrong password, SaltParseException → false for invalid hash
• IdentityTokenServiceTests — constructor rejects null/empty/whitespace key, Sign → two-part base64url token, Verify roundtrips userId+role, rejects tampered/wrong-key/expired tokens
• HooklineUserTests — HasAtLeast(role) hierarchy (Owner ≥ Admin ≥ Member), Anonymous/System/Authenticated() factory invariants

YouTubeUploads.Tests additions:

• SlackMrkdwnTests — entity expansion (<@U|name> → @name, <#C|ch> → #ch, <!here> → @here, <url|label> → label (url)), &/</> decode, angle-bracket stripping, emoji shortcode pass-through
• SlackEmojiTests — known shortcode → Unicode, :skin-tone-N: → stripped, unknown codes pass through, non-emoji colons like 16:9 unchanged
• ProgressTrackerTests — Set/Get/Remove on ConcurrentDictionary, JobProgress.Percent clamping (0–100, BytesTotal=0 → 0)
• ApiUsageReportTests — Build() produces YouTube/Drive/Slack groups, metric summation across multiple clients, Slack sorted descending, scope label resolution

YouTubeComments.Tests additions:

• PacificTimeTests — Today() validity, TodayKey() format/roundtrip, StartOfToday()/UntilMidnight() bounds
• GoogleApiExceptionExtensionsTests — IsTransientStatus (5xx/429 = true, 4xx = false), HasReason case-insensitive matching, null-safe
• PollingFrequencyTests + ReplyScanFrequencyTests — ToCron/ToInterval enum mapping, ReplyScanFrequency.Off → null

Link to Devin session: https://app.devin.ai/sessions/31abc6001d334122b892687c2d8e6a41
Requested by: @skyp1nus

[devin-ai-integration]

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