Add Bugsink exception adapter

[saturnflyer]

Summary

• Adds a new Bugsink exception source adapter that fetches issues via Bugsink's REST API (/api/canonical/0/issues/), enabling Newshound to display errors from a separately-hosted Bugsink instance
• Introduces exception_source_config on Configuration — a generic hash for passing connection details (url, token) and adapter-specific options to HTTP-based adapters
• Validates all required config keys at once, reporting every missing key in a single error rather than failing on the first one. Required keys are exposed via Bugsink.required_keys for external use
• Existing adapters (ExceptionTrack, SolidErrors) are unaffected; they accept but ignore the config

Usage

Newshound.configure do |config|
  config.exception_source = :bugsink
  config.exception_source_config = {
    url: "https://bugs.example.com",
    token: ENV["BUGSINK_TOKEN"],
    project_id: 1
  }
end

Test plan

• All 133 existing + new specs pass
• Standard Ruby linter passes
• Coverage at 93.89%
• Manual test against a live Bugsink instance

🤖 Generated with Claude Code