feat: harden spec immutability

[Sh1bari]

Spec cleanup: immutable records and copy semantics

Build fails after actions/checkout was bumped from v6 to v7 in PR #948:
#948

[gemini-code-assist]

Code Review

This pull request introduces defensive copying and immutability for collection fields across several record classes and builders in the org.a2aproject.sdk.spec package using List.copyOf and Map.copyOf. The review feedback suggests further refining these changes by replacing @Nullable Boolean with a primitive boolean in MessageSendConfiguration and utilizing explicit canonical constructors in CancelTaskParams to avoid redundant null checks on record accessors when non-null default values are guaranteed.

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