fix: keep exporting spans after OpenCode instance disposal - #44

Summary

Fixes a lifecycle bug where Langfuse traces stopped exporting after OpenCode disposed and re-created the plugin instance in the same process.

OpenCode can re-create plugin instances when the effective runtime configuration changes, such as after a model or plugin configuration update. The plugin's dispose path shut down the process-wide OpenTelemetry tracer provider. Since that provider cannot be registered again in the same process, spans created after the plugin was re-created were silently dropped.

OpenCode 0.5.0 also requires the re-created plugin instance to reuse the existing Langfuse client. Otherwise, spans can be created through the process-wide provider while forceFlush operates on a different provider.

The plugin now:

• Flushes spans instead of shutting down the tracer provider during OpenCode 1 and OpenCode 2 plugin disposal.
• Reuses the Langfuse client while the same tracer provider remains registered.
• Re-creates the client when the global tracer provider has been replaced.
• Includes an integration regression test covering disposal and re-creation in the same process.

Verification

• pnpm install
• pnpm run format
• pnpm run format:check
• pnpm run build
• pnpm run lint
• pnpm run knip
• pnpm run test:integration
• 3 integration test files passed
• 32 integration tests passed