chore(main): release 1.2.0

[a2a-bot]

🤖 I have created a release beep boop

1.2.0 (2026-09-29)

Features

• rest: serve HTTP+JSON responses as application/a2a+json (#1274) (dc5a5da)
• server: add caching headers to the agent card endpoint (#1272) (83d7f5d)
• server: add multi-replica cluster mode (#1281) (942d621)
• server: add opt-in validation of message media types against the agent card (#1269) (8037b24)

Bug Fixes

• compare in-memory task timestamps numerically (#1233) (0d5473c)
• server: ignore unrecognized request fields and fix parse-error data shape (#1273) (83f1cf8)
• server: reject a message whose contextId disagrees with its task (#1270) (c25022f)
• server: reject terminal-task operations with UnsupportedOperationError (#1268) (6cce91b)
• server: send PushNotificationConfig.authentication as an Authorization header (#1271) (5751d31), closes #585

Documentation

• samples: add Agent Card signing sample (#1198) (f3ac824)

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[a2a-bot]

🤖 Created releases:

• v1.2.0

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