fix: enforce per-task push notification config limit

[ez-lbz]

What changed

1. Enforce a per-task push notification config limit

Problem: InMemoryPushNotificationConfigStore.setInfo maintained an unbounded List<TaskPushNotificationConfig> per task. A client could register unlimited push notification configs for one task; each config consumes memory and can trigger an outbound HTTP request on every task event, enabling resource exhaustion and amplified outbound traffic.

Fix (server-common/src/main/java/org/a2aproject/sdk/server/tasks/InMemoryPushNotificationConfigStore.java):

• Added MAX_PUSH_CONFIGS_PER_TASK = 100 (matching the Python SDK's cap).
• setInfo now counts only new configs against the limit: re-registering/updating an already-registered config ID remains allowed. When a genuinely new config would exceed the limit, setInfo throws InvalidParamsError (code -32602), the same error family already used for push-config validation, which transports surface as a client error.

Fix (server-common/src/test/java/org/a2aproject/sdk/server/tasks/InMemoryPushNotificationConfigStoreTest.java):

• testSetInfoAtLimitExactlyAllowed — the 100th config still succeeds.
• testSetInfoRejectsExceedingPerTaskLimit — the 101st distinct config throws InvalidParamsError and the store is unchanged.
• testSetInfoUpdateExistingConfigAtLimitAllowed — updating an existing config at the limit is still allowed.

Behavior change: registering more than 100 distinct push notification configs for a single task now fails with InvalidParamsError instead of succeeding. Updates to existing configs are unaffected.

Testing

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