feat: Add PR description and file changes to LLM context

Description

Add PR description (body) and list of files changed to the LLM context to provide richer information for AI-powered analysis.

Context

This was suggested in PR #2292 review comment: https://github.com/openshift-pipelines/pipelines-as-code/pull/2292#discussion_r2468174103

Currently, the LLM context includes basic PR information (number, title, labels) but not:

• PR description/body
• List of files changed in the PR

Proposed Changes

1. Extend Event struct (pkg/params/info/events.go):

   • Add PullRequestBody string field
   • Add PullRequestFiles []string field (or more detailed file change info)

2. Update provider implementations to fetch this data:

   • GitHub provider
   • GitLab provider
   • Gitea provider
   • Bitbucket providers

3. Update context assembler (pkg/llm/context/assembler.go):

   • Include PR body in buildPRContent() method
   • Include file changes list in the context

Benefits

• Better context for LLM to understand what the PR is trying to accomplish
• File changes can help identify which areas of code might be related to failures
• More comprehensive analysis of test failures in relation to code changes

Priority

This is a future enhancement and not blocking for the initial LLM analysis feature.

Start by reading pkg/params/info/events.go and pkg/llm/context/assembler.go, especially buildPRContent(), then inspect the GitHub, GitLab, Gitea, and Bitbucket provider implementations. Done means the event carries the PR body and changed-file list, each provider supplies them, and the assembler includes both in the LLM context.