Rename the X-Otari-Code-Execution header to Otari-Code-Execution before it ships

[peteski22]

Problem

#1366 added a request header, X-Otari-Code-Execution, that picks who runs a provider-native code-execution declaration. It is the constant CODE_EXECUTION_HEADER in api/routes/_tools.py, read by the chat, messages and responses routes.

RFC 6648 (BCP 178) says creators of new parameters "SHOULD NOT prefix their parameter names with 'X-' or similar constructs": https://datatracker.ietf.org/doc/html/rfc6648. Otari's other headers already follow it: Otari-Key, Otari-Router, Otari-Router-Task, Otari-Conversation-Id.

The header has not shipped: no release tag contains #1366. So it can be renamed now, with no deprecation.

Proposal

Rename it to Otari-Code-Execution. The literal appears in these files on main:

• src/gateway/api/routes/_tools.py, src/gateway/api/routes/_pipeline.py
• src/gateway/core/config.py and src/gateway/services/tenancy/workspace_code_execution_policy_service.py (field descriptions)
• src/gateway/AGENTS.md, docs/tools.md
• tests/integration/test_code_execution_executor.py
• demo/code-exec/plot_with_openai_sdk.py
• the generated docs/public/openapi.json and web/src/client/schema.ts

Then regenerate the public artifacts and the dashboard client, as "Generated Artifacts" in AGENTS.md lists them.

Acceptance

• git grep "X-Otari-Code-Execution" finds nothing.
• A test sends Otari-Code-Execution on each of chat, messages and responses, and the choice applies.
• make openapi-check, make postman-check and the dashboard client drift check pass.

Part of #1470.