fix: mirror current Facebook and Threads crossposting

[subzeroid]

Summary

• mirror instagrapi 2.18.15 crossposting behavior for the current mobile GraphQL WWW transport and connected-services destination discovery
• preserve eligibility, audience, partial overrides, and Reel validation payload behavior, including 114-character destination IDs
• distinguish missing linked destinations from network and protocol errors, and strengthen authenticated live account selection

Upstream sync: subzeroid/instagrapi#2768
Related upstream reports: subzeroid/instagrapi#2754 and subzeroid/instagrapi#2755

Test plan

• 800 passed, 13 skipped, and 35 subtests in the local CI regression target
• 85 passed and 3 subtests in the focused crossposting suite
• Ruff check and format, Bandit, pre-commit, strict MkDocs, package build, and diff check passed
• mypy error count is unchanged from main under the same local environment: 221 vs 221
• authenticated live transport checks: 4 passed; 3 end-to-end crosspost cases skipped after the account pool had no linked Facebook or Threads destination available