Add billing context to LLM telemetry traces (#9745)

* feat: add billing context to LLM telemetry traces

Add billing metadata (plan type, cost source, customer ID) to LLM traces in ClickHouse for cost analytics and attribution.

**Data Flow:**
- Cloud-API: Extract billing info from subscription in rate limiting, set x-billing-* headers
- Core: Parse headers into BillingContext object via dependencies
- Adapters: Flow billing_context through all LLM adapters (blocking & streaming)
- Agent: Pass billing_context to step() and stream() methods
- ClickHouse: Store in billing_plan_type, billing_cost_source, billing_customer_id columns

**Changes:**
- Add BillingContext schema to provider_trace.py
- Add billing columns to llm_traces ClickHouse table DDL
- Update getCustomerSubscription to fetch stripeCustomerId from organization_billing_details
- Propagate billing_context through agent step flow, adapters, and streaming service
- Update ProviderTrace and LLMTrace to include billing metadata
- Regenerate SDK with autogen

**Production Deployment:**
Requires env vars: LETTA_PROVIDER_TRACE_BACKEND=clickhouse, LETTA_STORE_LLM_TRACES=true, CLICKHOUSE_*

🐾 Generated with [Letta Code](https://letta.com)

Co-Authored-By: Letta <noreply@letta.com>

* fix: add billing_context parameter to agent step methods

- Add billing_context to BaseAgent and BaseAgentV2 abstract methods
- Update LettaAgent, LettaAgentV2, LettaAgentV3 step methods
- Update multi-agent groups: SleeptimeMultiAgentV2, V3, V4
- Fix test_utils.py to include billing header parameters
- Import BillingContext in all affected files

* fix: add billing_context to stream methods

- Add billing_context parameter to BaseAgentV2.stream()
- Add billing_context parameter to LettaAgentV2.stream()
- LettaAgentV3.stream() already has it from previous commit

* fix: exclude billing headers from OpenAPI spec

Mark billing headers as internal (include_in_schema=False) so they don't appear in the public API.
These are internal headers between cloud-api and core, not part of the public SDK.

Regenerated SDK with stage-api - removes 10,650 lines of bloat that was causing OOM during Next.js build.

* refactor: return billing context from handleUnifiedRateLimiting instead of mutating req

Instead of passing req into handleUnifiedRateLimiting and mutating headers inside it:
- Return billing context fields (billingPlanType, billingCostSource, billingCustomerId) from handleUnifiedRateLimiting
- Set headers in handleMessageRateLimiting (middleware layer) after getting the result
- This fixes step-orchestrator compatibility since it doesn't have a real Express req object

* chore: remove extra gencode

* p

---------

Co-authored-by: Letta <noreply@letta.com>

tests/test_utils.py

@@ -24,6 +24,9 @@ def test_get_headers_user_id_allows_none():
         letta_v1_agent=None,
         letta_v1_agent_message_async=None,
         modal_sandbox=None,
+        billing_plan_type=None,
+        billing_cost_source=None,
+        billing_customer_id=None,
     )
     assert isinstance(headers, HeaderParams)
 
@@ -40,6 +43,9 @@ def test_get_headers_user_id_rejects_invalid_format():
             letta_v1_agent=None,
             letta_v1_agent_message_async=None,
             modal_sandbox=None,
+            billing_plan_type=None,
+            billing_cost_source=None,
+            billing_customer_id=None,
         )
 
 
@@ -54,6 +60,9 @@ def test_get_headers_user_id_accepts_valid_format():
         letta_v1_agent=None,
         letta_v1_agent_message_async=None,
         modal_sandbox=None,
+        billing_plan_type=None,
+        billing_cost_source=None,
+        billing_customer_id=None,
     )
     assert headers.actor_id == "user-123e4567-e89b-42d3-8456-426614174000"