feat: integrate pr-test-oracle for AI-powered test recommendations

[myakove]

Description

Integrate pr-test-oracle into the webhook server to provide AI-powered test recommendations for pull requests. The oracle analyzes PR diffs and recommends which tests to run.

Requirements

• New test-oracle config section (global + per-repo) with:
  • server-url (required)
  • ai-provider (required): claude | gemini | cursor
  • ai-model (required)
  • test-patterns (optional)
  • triggers (optional, default: [approved]): approved, pr-opened, pr-synchronized
• /test-oracle comment command (always works when configured)
• Health check before calling oracle - post PR comment if server is down
• HTTP/network errors from /analyze are logged only (no PR comment)
• Never break webhook processing flow

Deliverables

• Add COMMAND_TEST_ORACLE_STR constant and config schema
• Create webhook_server/libs/test_oracle.py module with call_test_oracle() helper
• Add /test-oracle comment command to IssueCommentHandler
• Call test oracle on PR approval in PullRequestReviewHandler
• Call test oracle on PR opened/synchronized in PullRequestHandler
• Update example configs with test-oracle examples
• Full test suite passing with >= 90% coverage

Design

See docs/plans/2026-02-28-test-oracle-integration.md