Set CAPTURE_MESSAGE_CONTENT_IN_SPANS default to false

[r4inee]

Link to Issue or Description of Change

• Closes: Telemetry: Default message content capture to false #1143

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.

Solution:
Tracing.java — The default value of the ADK_CAPTURE_MESSAGE_CONTENT_IN_SPANS environment variable should default from true → false.