feat(integrations): Add POST endpoint for coding agent launch

[jennmueng]

• Add POST method to OrganizationCodingAgentsEndpoint for launching coding agents
• Support launching agents for multiple repositories from autofix solutions
• Include branch name sanitization and stores coding agent state to Seer on trigger

Context

This PR completes the coding agent framework by adding the POST endpoint that enables launching coding agents from the Seer UI. The endpoint:

1. Validates the integration and autofix state
2. Extracts repositories from the autofix solution
3. Generates appropriate prompts based on trigger source
4. Launches coding agents for each repository
5. Stores the agent state in Seer for tracking
6. Handles errors gracefully and continues processing other repos

The endpoint supports both root_cause and solution trigger sources, and includes comprehensive error handling and logging.

Stacked on: #97986

Architecture

• POST /api/0/organizations/{org}/integrations/coding-agents/
• Feature flagged behind organizations:seer-coding-agent-integrations
• Integrates with Seer API for prompt generation and state storage
• Multi-repository support with graceful error handling

[cursor]

Bug: Missing Default in next() Call Causes Crash

The _extract_repos_from_solution method uses next() without a default value when searching for the "solution" step. If this step is absent from autofix_state.steps, a StopIteration exception is raised, causing the POST endpoint to crash.

[seer-by-sentry]

Potential bug: Unsafe dictionary access on solution_item["relevant_code_file"] may cause a KeyError. Frontend types show this field is optional, but the backend doesn't handle its absence, risking a server crash.

• Description: The code directly accesses solution_item["relevant_code_file"] within a loop. Evidence from corresponding frontend TypeScript types (AutofixSolutionTimelineEvent) indicates that the relevant_code_file field is optional. If the Seer API, which provides this data, returns a solution_item where this key is missing, the direct dictionary access will raise an unhandled KeyError. This will cause the API endpoint to crash and return a 500 error, preventing the extraction of repository information from the autofix state.

• Suggested fix: Use the .get() method for safe dictionary access to prevent a KeyError when the relevant_code_file key is missing. The code should be changed to solution_item.get("relevant_code_file") and include a check for the result before attempting to access nested keys like repo_name.
  _{severity: 0.65, confidence: 0.9}

_{Did we get this right? 👍 / 👎 to inform future reviews.}