FEAT: Add groupDescription, sync Unserved with 3 new codes

[kishorereddy]

Overview

1. New groupDescription field: a runtime-readable explanation for each status category.
2. KDoc cleanup: class docs now point at groupDescription instead of restating it.
3. 3 new codes: DEGRADED, LEGAL_BLOCK, ABORTED added to Unserved, closing a gRPC gap.

Ticket(s)

N/A

Link(s)

N/A

Example(s)

Restricted.DENIED.groupDescription
// "An access-control failure — the caller is not permitted to perform this action."

// ABORTED now maps directly to gRPC's own ABORTED code
CodesToGrpc().toCode(Unserved.ABORTED) // 10, was an honest null gap before

Dependencies

None.

Design

1. groupDescription: added to Passed and Failed, same pattern group already uses —
   makes each category's meaning a real runtime value instead of only living in a KDoc comment.
2. KDoc trimmed: most subtypes now just point at groupDescription. Kept the genuinely useful
   extra detail on Excluded (the SKIPPED/DISCARDED naming note) and Unserved (the example
   list), rather than deleting everything uniformly.
3. DEGRADED: a dependency is degraded, some calls may be refused.
4. LEGAL_BLOCK: access restricted for legal/regulatory reasons. Maps to HTTP 451.
5. ABORTED: operation aborted before completing, retrying may succeed. Maps directly to
   gRPC's own ABORTED (10) — gRPC now has full 0-16 coverage, no more gaps.
6. Registry is now 57 codes total.

Notes

1. Left Excluded.CANCELLED's message wording and LEGAL_BLOCK's category placement alone —
   both still open questions, not settled enough to bundle into this change.

Pending

None.

Tests

1. Added tests for groupDescription (non-blank, distinct per category, consistent per subtype).
2. Added tests for the 3 new codes and their protocol mappings, including the deliberate
   category-default fallthrough for DEGRADED/LEGAL_BLOCK on gRPC.
3. Replaced the old "ABORTED maps to null" test with one confirming the real mapping.
4. Full ktlintCheck + detekt + jvmTest + jsNodeTest run clean.