feat(log-enrichment): add custom session ID attribute for enriched spans

[AaronProbha18]

Summary

This PR stops storing session IDs in the OpenTelemetry semantic convention attribute gen_ai.agent.name and instead records them in a repository-specific custom attribute, agentevals.session.id.

Previously, the log enrichment pipeline wrote the session ID into gen_ai.agent.name, overwriting legitimate agent names from SDK and OTLP telemetry. This caused downstream metadata extraction to incorrectly report the session ID as the agent name.

Changes

• Added the new custom telemetry attribute agentevals.session.id.
• Replaced all session ID writes in the log enrichment pipeline to use agentevals.session.id.
• Factored the duplicated session ID injection logic into a shared helper used by both enrichment paths.
• Preserved gen_ai.agent.name for genuine agent names supplied by telemetry.
• Updated log enrichment tests to verify that session IDs are written to the new attribute.