Adds OpenTelemetry baggage propagation for session descriptions, enabling contextual metadata to flow through distributed traces.

Changes

• Constants: Added GEN_AI_SESSION_DESCRIPTION_KEY = 'gen_ai.session.description' to OpenTelemetry constants
• BaggageBuilder: Added sessionDescription(value: string | null | undefined) fluent setter using existing set helper
• Span Propagation: Included GEN_AI_SESSION_DESCRIPTION_KEY in GENERIC_ATTRIBUTES array for automatic span attribute enrichment
• Tests: Added coverage for baggage entry setting and span attribute propagation
• Sample: Demonstrated usage in basic-agent-sdk-sample

Usage

const baggageScope = new BaggageBuilder()
  .tenantId(tenantInfo.tenantId)
  .agentId(agentInfo.agentId)
  .sessionDescription('Initial onboarding session')
  .build();

baggageScope.run(async () => {
  // Session description propagates to all spans in this context
});

This pull request was created as a result of the following prompt from Copilot chat.

Simple change: add session description baggage support.

Scope:

1. Add OpenTelemetry constant GEN_AI_SESSION_DESCRIPTION_KEY = 'gen_ai.session.description' in packages/agents-a365-observability/src/tracing/constants.ts near existing GEN_AI_AGENT_DESCRIPTION_KEY.
2. Add fluent setter sessionDescription(value: string | null | undefined): BaggageBuilder to packages/agents-a365-observability/src/tracing/middleware/BaggageBuilder.ts (adjacent to sessionId and agentDescription methods). Reuse private set helper.
3. Update propagation list in packages/agents-a365-observability/src/tracing/processors/util.ts (array of consts) to include GEN_AI_SESSION_DESCRIPTION_KEY so it becomes a span attribute.
4. Tests:
   • Extend tests/observability/core/BaggageBuilder.test.ts with a test verifying builder.sessionDescription('My session desc') sets baggage entry.
   • Extend tests/observability/core/SpanProcessor.test.ts to assert a span created under baggage containing GEN_AI_SESSION_DESCRIPTION_KEY has the attribute (similar pattern to other baggage keys).
5. Optional sample update: In tests-agent/basic-agent-sdk-sample/src/agent.ts add chained .sessionDescription('Initial onboarding session').

Keep description minimal per request. Do not modify unrelated code.

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