feat: block can-be-merged when unresolved review conversations exist

[rnetser]

Description

Add a new config option required-conversation-resolution that, when enabled, fails the can-be-merged check if the PR has unresolved review threads.

GitHub's REST API doesn't expose thread resolution status — only the GraphQL API v4 has reviewThreads { isResolved }. The codebase already has httpx as a dependency, so we'll use it for the GraphQL call.

Requirements

• New required-conversation-resolution boolean config at global + repository level in schema
• GraphQL helper method get_unresolved_review_threads() on GithubWebhook class
• Integration into check_if_can_be_merged() in PullRequestHandler
• Threads that are resolved or outdated are excluded
• Failure message shows file:line for up to 5 unresolved threads

Deliverables

• Add required-conversation-resolution to schema.yaml (global + repo level)
• Load config in github_api.py _repo_data_from_config()
• Add get_unresolved_review_threads() GraphQL method to GithubWebhook
• Integrate check in check_if_can_be_merged() (parallel with existing gather)
• Add tests for GraphQL helper and handler integration
• Update test_schema_validator.py and test_config_schema.py
• Add examples in examples/config.yaml and examples/.github-webhook-server.yaml