feat: handle usageUpdate events and populate UsageMetadata in AgentResponse

[glaforge]

Improvements & Bug Fixes

• Handle usageUpdate events: Process dedicated usageUpdate and top-level usageMetadata OutputEvents sent by localharness, ensuring UsageMetadata is populated in AgentResponse.
• Session & Turn Accounting: Track cumulative token usage across the session and compute per-turn delta via subtract(). Expose agent.getTotalUsage() and agent.getTrajectoryUsages().
• Wire Compatibility: Parse both camelCase and snake_case field variants (e.g. promptTokenCount / prompt_token_count) for token metrics and modality breakdown.
• Ergonomic Aliases: Add response.usage() convenience alias matching SDK documentation.
• UsageMetadata Arithmetic: Enhance add() and subtract() identity handling for zero token counts to preserve modality details.
• Testing: Add fast unit tests (UsageMetadataParsingTest) and strengthen ObservabilityTest assertions against live localharness.