feat(langgraph-cloud): configure trusted CORS origins

[John Kennedy (jkennedyvz)]

Summary

• adds config.corsAllowOrigins to the langgraph-cloud chart
• renders trusted origins as CORS_ALLOW_ORIGINS on the Agent Server API deployment
• rejects wildcard origins and values containing paths or trailing slashes
• documents the value and adds Helm unit coverage

Motivation

Supports the self-hosted migration for langchain-ai/langgraph-api#3135. Direct self-hosted browser clients using cookie authentication need explicit trusted origins once credentialed wildcard CORS is disabled.

Example:

config:
  corsAllowOrigins:
    - https://langsmith.example.com

Multiple entries are rendered as the comma-separated format expected by Agent Server.

Validation

• helm lint charts/langgraph-cloud
• rendered no-value and multi-origin configurations with Helm 3.12.1
• verified wildcard and path-bearing origins fail rendering
• git diff --check

The Helm unittest plugin could not be installed locally because its release checksum download failed; CI runs the added suite with the repository-pinned plugin.

Release Notes: Added config.corsAllowOrigins to configure explicit trusted browser origins for credentialed Agent Server requests.