fix(graph): make PostgresSaver latest checkpoint deterministic

[GaoSSR]

Describe what this PR does / why we need it

PostgresSaver currently orders checkpoints by saved_at when loading checkpoint history and the latest checkpoint. In concurrent writes, two checkpoints can share the same timestamp, so the database can return either row first.

This PR makes PostgreSQL checkpoint ordering deterministic by using an auto-increment sequence column, matching the MySQL saver behavior.

Does this pull request fix one issue?

NONE

Describe how you did it

• Add a checkpoint_seq sequence column to GraphCheckpoint and create it for existing tables when tables are initialized.
• Order checkpoint history and latest-checkpoint queries by checkpoint_seq DESC instead of saved_at DESC.
• Add an index for checkpoint lookups by thread and sequence.
• Add regression coverage that forces tied saved_at values and verifies list/get return the newest inserted checkpoint.

Describe how to verify it

• CI=true ./mvnw -pl spring-ai-alibaba-graph-core -Dtest=PostgresSaverTest#testPostgresSaverOrdersCheckpointsByInsertSequenceWhenSavedAtTies test
• CI=true ./mvnw -pl spring-ai-alibaba-graph-core -Dtest=PostgresSaverTest test
• git diff --check

Special notes for reviews

The sequence is only used for deterministic ordering. Existing saved_at values remain unchanged, and CreateOption.CREATE_NONE still does not modify existing schemas.

[GaoSSR]

CI note: the test job is failing in spring-ai-alibaba-agent-framework on two timeout cancellation tests:

• AgentToolNodeAsyncExecutionTest$StateUpdateOnTimeoutTests.stateUpdates_shouldBeCleared_onTimeoutWithCancellableTool
• CancellableAsyncToolCallbackTest$TimeoutScenarioTests.tool_shouldStopGracefully_whenCheckingCancellation

The same failures are present on a recent main Build and Test run: https://github.com/alibaba/spring-ai-alibaba/actions/runs/27903896284

The spring-ai-alibaba-graph-core module passed in this PR run.