Fix PullMerge() failure

[khanaffan]

imodel-native: iTwin/imodel-native#1162

Issue Summary

1. Initial Changeset Push:
   OpenSite+ modified the GCS and pushed a changeset.

2. Schema Import by Civil Connector:
   Civil Connector imported a schema and made additional GCS changes.

3. PullMerge Failure in OpenSite+:
   OpenSite+ (with no local changes) attempted a PullMerge(), which failed due to the following sequence:

   • A schema changeset was applied.
   • TxnManager disables tracking during applyChangeset(), but afterward, it recomputes ec_cache_* tables, which are tracked.
   • pullMergeEnd() calls saveChanges(), creating a transaction.
   • A subsequent changeset apply detects a rebase is needed.
     • During rebase, it tries to undo the transaction containing ec_cache_* changes.
     • This fails with a FOREIGN_KEY_CONSTRAINT error.
     • The failure occurs because ec_cache_* tables reference ec_class via foreign keys. Reverting the cache tables leads to missing parent ec_class entries.

---

Solution

1. Exclude ec_cache_* from Tracking:
   These tables are derived and can be safely recomputed, so they should not be tracked.

2. Disable NOFKCONSTRAINT Flag for Schema Changesets:
   Since ec_cache_* tables have FK relationships to ec_class and ec_table, cascading deletes must be allowed. This ensures that when primary tables are modified, related cache entries are also cleaned up properly.

---

Additional Safeguard

• A warning has been added to flag any local changes detected during PullMergeEnd(), as ideally, there should be none.

[Copilot]

Pull Request Overview

This PR addresses a failure in PullMerge() by adding validation and tooling around changeset application.

• Adds a standalone test to verify that applying a schema changeset with no local edits does not create pending transactions.
• Introduces exportAsJson in SqliteChangesetReader to help debug and inspect changesets.
• Updates the change metadata file to document this fix.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 4 comments.

File	Description
core/backend/src/test/standalone/ApplyChangeset.test.ts	New test verifying no local changes after pull merge
core/backend/src/SqliteChangesetReader.ts	Added exportAsJson helper for dumping changesets
common/changes/@itwin/core-backend/affank-fix-pullmerge_2025-06-27-11-56.json	Metadata entry for this PullMerge fix

Comments suppressed due to low confidence (1)

core/backend/src/SqliteChangesetReader.ts:434

• The new exportAsJson utility is untested; consider adding unit tests to validate JSON structure, filtering logic, hashing of blobs, and indentation.

  public exportAsJson(

[Copilot]

The test starts IModelHost but never shuts it down; consider adding an after hook to call await IModelHost.shutdown() and ensure clean teardown.