fix(sdk): report MigrationCooldownNotMet as a skip, not a failure (#725)

[Iker2522]

Summary

#557 lengthened migrate_adapter's MIN_LEDGER_GAP cooldown from ~1 minute to ~1 day. The migration keeper didn't special-case the resulting MigrationCooldownNotMet rejection, so every hourly run during that ~1-day window produced an HTTP 500 and a failure log entry — undermining the timelock's purpose of giving observers a calm window to watch for a malicious begin_migration.

What changed

Added isMigrationCooldownError (packages/stellar-sdk-helpers/src/keeper-tx.ts), matching the contract's MigrationCooldownNotMet = 20 in the raw simulation error text — same message-text approach isStaleAdapterError already uses, since withKeeperRetry's wrapping loses the original error's type.
migration-keeper.ts now checks it before the existing isStaleAdapterError check and reports a skip instead of a failure.
api/v1/keepers/[action].ts needs no changes — its failures.length > 0 ? 500 : 200 status logic already does the right thing once the count is accurate.
Fixed a stale comment describing the bug as intended behavior, and updated apps/docs/operations/migration-keeper.md, which already documented this exact gap as a tracked follow-up.

Test plan

pnpm typecheck clean
75/75 tests pass (2 new: a unit test for isMigrationCooldownError, and an integration test asserting the full runMigrationKeeper path reports skipped/not failures/no retry for a #20 simulation rejection)
apps/docs vitepress build succeeds
prettier --check passes on every changed file

Closes #725

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[Iker2522]

Hi! This PR fixes issue #725 — MigrationCooldownNotMet now reports as a skip instead of a failure, so the keeper stops paging on expected behavior during the ~1-day cooldown window. All three acceptance criteria are covered: detection + skip handling, updated docs, and test coverage (unit + integration). Ready for review whenever you get a chance — let me know if anything needs adjusting!

[collinsezedike]

@Iker2522 the code here is correct, I checked it against the actual contract source and traced the fix through the loop, lease release, and both the unit and integration tests. Nothing to change there.

CI is failing on Commit Messages and PR Title because neither has a Conventional Commits type prefix. Please read CONTRIBUTING.md's "Commit Convention" section and update both to something like fix(sdk): report MigrationCooldownNotMet as a skip, not a failure.

[collinsezedike]

@Iker2522 Thank you for the contribution. Feel free to pick up another open issue whenever you're ready.

[collinsezedike]

@Iker2522 If you found working on this useful, a star on the repo goes a long way in helping the project grow.