fix: guard limitTaskHistory against negative historyLength

[ez-lbz]

What changed

1. Guard limitTaskHistory against negative historyLength

Problem: The guard in limitTaskHistory only checked historyLength >= task.history().size(), which does not catch negative values. A negative historyLength fell through to subList(size - historyLength, size) — i.e. subList(size + 1, size) — throwing IndexOutOfBoundsException. While TaskQueryParams rejects negative values at construction today, limitTaskHistory lacked defensive validation and would crash if a negative value ever reached it.

Fix (server-common/src/main/java/org/a2aproject/sdk/server/requesthandlers/DefaultRequestHandler.java):

• Extended the early-return guard to historyLength < 0, so a negative value returns the task untouched instead of crashing.
• Semantics are aligned with the Python SDK (apply_history_length): negative → history untouched; 0 → empty history; N >= history size → no limiting. The existing 0 → empty history behavior is preserved (unlike a <= 0 guard, which would have broken it).

Fix (server-common/src/test/java/org/a2aproject/sdk/server/requesthandlers/DefaultRequestHandlerTest.java):

• testLimitTaskHistoryNegativeHistoryLengthReturnsTaskUnchanged — invokes the private limitTaskHistory via reflection with -1 and asserts no exception is thrown and the task is returned unchanged.
• testOnGetTaskHistoryLengthLimitsHistory — historyLength=2 returns only the 2 most recent messages.
• testOnGetTaskHistoryLengthZeroReturnsEmptyHistory — locks in that historyLength=0 still returns an empty history.

Behavior change: none for valid inputs (>= 0). Negative historyLength previously crashed; it now returns the task unchanged.

Testing

• mvn -pl server-common test — 450 tests run, 0 failures, 0 errors, 0 skipped (BUILD SUCCESS), including the 3 new regression tests.