Fix real Intercom API compatibility failures

[uffejaeger]

Summary

This batch fixes the compatibility failures found against the real Intercom API:

• accepts live wire variants for pagination, IDs, timestamps, admin avatars, attachment dimensions, conversation statistics, and contact-company association envelopes
• corrects generated request types for real contact IDs and preserves typed API errors when request IDs are not UUIDs
• makes article-draft empty responses semantic SDK errors instead of generated JSON failures
• falls back to the documented contact representation when the dedicated merge-history route returns 404
• adds deterministic regression coverage plus opt-in read-only and fixture-backed real-API suites

Issues fixed

Closes #76
Closes #77
Closes #78
Closes #79
Closes #80
Closes #82
Closes #83
Closes #84
Closes #87
Closes #88
Closes #89
Closes #90
Closes #94
Closes #98
Closes #99
Closes #100
Closes #101

Investigated live-API reports

• Investigate Audiences.List IDs not usable with Audiences.Get #85, Investigate content snippet IDs from list not usable with GetSnippet #86, Investigate content import-source IDs from list not usable with GetContentImportSource #91, Investigate external-page IDs from list not usable with GetExternalPage #92, Investigate help-center redirect IDs from list not usable with Get #93, Investigate newsfeed IDs from ListFeeds not usable with RetrieveFeed #95, and Investigate ListFeedItems with real newsfeed IDs from ListFeeds #96: SDK routes match Intercom's published routes; the dev workspace has no resources for list-to-detail verification, so the live tests skip with an explicit reason.
• Teams.Metrics returns 503 for a valid listed team #97: Intercom returns a typed 503 after retries; the endpoint requires the realtime_monitoring workspace feature.
• Company/contact association lists omit successfully attached fixture resources #101: the live API returns contact-company lists in a data envelope although the published model uses companies; the SDK now accepts both. The fixture passed after Intercom's asynchronous association index converged.

Verification

• make pre-push
  • formatting, vet, staticcheck, and build
  • coverage: 99.9%
  • generated-code freshness
  • public API compatibility with v0.2.0
  • vulnerability scan
• go test -tags=live -run '^TestLiveAPIReadCompatibility$' -count=1 -v
• go test -tags=live -run '^TestLiveAPIFixtureCompatibility$' -count=1 -v
  • creates and cleans up a scoped contact, company, note, conversation, and association
  • Allow Intercom contact IDs in ConversationCreate.From.Id #82, Investigate Contacts.ListMergeHistory returning 404 for valid contacts #83, Allow real Intercom contact IDs in Contacts.CreateNote #99, and Company/contact association lists omit successfully attached fixture resources #101 passed against the live API
• go test -tags=live -run '^TestLiveAPIContentExportJobCompatibility$' -count=1 -v
  • Accept content-export job identifiers in Workspace.JobStatus #98 passed with a real export identifier; the export was cancelled after verification

No access token or customer data is committed.