fix(er-position): persist the new position, not the old record

[Aias00]

Related issue

Closes #2160

Summary

ERPositionStorage.savePosition found the existing record p (carrying the OLD position) and called update(p), discarding param's new position. The ER diagram layout could never be updated for an existing (dataSourceId, databaseName, schemaName) key. Fixed by copying param's position into p before updating: p.setPosition(param.getPosition()); update(p);.

Verification

• mvn compile -> BUILD SUCCESS.

Contributor declaration

• I linked the Issue that defines this change.
• I tested the affected behavior and reported the actual results above.
• I did not include credentials, private data, or generated build output.
• I disclosed substantial AI assistance below, or this PR contains no substantial AI-generated code.

AI assistance: The fix, verification, and PR description were produced with Claude Code assistance.

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[openai0229]

Updating the stored position on the existing record is the right fix, but this PR is not ready to merge without persistence regression coverage.

Please add an automated storage test that saves an ER position, saves a different position for the same logical key, reloads it, and asserts that the new position is returned and no duplicate record was inserted. Rebase onto the latest main before requesting another review.

[openai0229]

The persistence fix still lacks a save-update-reload regression test proving the new position replaces the old record without duplication. The current head does not address the detailed review comment already posted, so this remains blocked pending an updated commit and re-review.

[Aias00]

Rebased onto latest main. Added 4 regression tests in ERPositionStorageTest:

1. savePositionInsertsNewRecord — first save creates one record
2. savePositionReplacesExistingPosition — second save for same key replaces position, no duplication
3. savePositionDoesNotAffectOtherKeys — updating one key doesn't affect other keys
4. getPositionReturnsNullForMissingKey — returns null for unknown key

Tests run: 4, Failures: 0.

[openai0229]

The storage path now persists the incoming ER position and the regression test verifies the value after a fresh reload from an isolated temporary file. File injection and lazy singleton initialization keep the production path intact while making the behavior testable. Storage and upstream module tests pass. Maintainer review complete; awaiting required checks before merge.