fix(graph): avoid mutating append inputs

[Gujiassh]

Describe what this PR does / why we need it

This keeps AppendStrategy.apply(oldValue, newValue) side-effect free when oldValue is already a List and newValue is a single non-collection value.

Today that branch appends directly into the incoming oldList and returns the same instance. Any caller that still holds the original list reference can observe the mutation, which leaks state across history snapshots and checkpointed branches.

Does this pull request fix one issue?

Fixes #4757

Describe how you did it

• change the single-value append branch to copy oldList into a new ArrayList
• append the new value to the copied list instead of mutating the original reference
• add a focused regression test that proves oldList stays unchanged while the returned list contains the appended value

Describe how to verify it

• ./mvnw -pl spring-ai-alibaba-graph-core -Dtest=AppendStrategyMutationRegressionTest test
• git diff --check

Special notes for reviews

This is intentionally scoped to the single-value append path only. Existing list/list merge behavior and remove-identifier handling are left unchanged.