Telemetry: Default message content capture to false

### Is your feature request related to a specific problem?
LLM request/response content and tool arguments/responses will no longer be captured in OpenTelemetry spans unless explicitly opted in.

ADK should provide a privacy-first default to avoid UGC being accidently extracted, aligning with OTEL conventions.

### Describe the Solution You'd Like
Tracing.java — The default value of the ADK_CAPTURE_MESSAGE_CONTENT_IN_SPANS environment variable should default from true → false.

### Impact on your work
Various systems need to explicitly set the environment variable to false in order to opt-out of the feature.

### Willingness to contribute
Yes