Replace X-Request-Id with the trace ID and retire the guard suite

Step 6: downstream_client() no longer sends X-Request-Id, and request_id
is gone from the internal request bodies, models, function signatures
and log lines; correlation now travels as traceparent. The gateway's
public request_id stays (the contract tests require it) but is now the
trace ID, 32 hex digits and still a valid UUID, so the value the
frontend shows can be pasted into Tempo.

Step 7: delete tests/test_no_observability.py. It guarded the Phase 0
"no observability" state, which Phase 3 ends by design.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

services/ledger/app.py

@@ -36,7 +36,6 @@ class CreateTransactionRequest(BaseModel):
     currency: str
     merchant_name: str
     category_id: int | None = None
-    request_id: str = "unknown"
 
     @field_validator("amount_minor")
     @classmethod
@@ -49,7 +48,6 @@ class CreateTransactionRequest(BaseModel):
 
 class SettleRequest(BaseModel):
     risk_score: float
-    request_id: str = "unknown"
 
 
 @app.exception_handler(Exception)
@@ -93,7 +91,7 @@ async def list_accounts():
 
 @app.post("/internal/transactions")
 async def create_transaction(body: CreateTransactionRequest):
-    logger.info(f"processing tx for account {body.account_id}, amount {body.amount_minor} request_id={body.request_id}")
+    logger.info(f"processing tx for account {body.account_id}, amount {body.amount_minor}")
 
     starved = await chaos.pool_starvation_enabled()
     poison_enabled = await chaos.poison_data_enabled()
@@ -153,7 +151,7 @@ async def settle(tx_id: str, body: SettleRequest):
             use_lock = row_lock and bool(account.is_hot)
             balance = await repository.settle_transaction(session, tx=tx, risk_score=body.risk_score, use_row_lock=use_lock)
 
-    logger.info(f"settled transaction {tx_id} request_id={body.request_id}")
+    logger.info(f"settled transaction {tx_id}")
     return {"status": "settled", "balance_minor": balance}
 
 
@@ -167,5 +165,5 @@ async def reject(tx_id: str, body: SettleRequest):
             return JSONResponse(status_code=404, content={"error": "transaction not found"})
         balance = await repository.reject_transaction(session, tx=tx, risk_score=body.risk_score)
 
-    logger.info(f"rejected transaction {tx_id} request_id={body.request_id}")
+    logger.info(f"rejected transaction {tx_id}")
     return {"status": "rejected", "balance_minor": balance}