Report error when API silently fails to add review comment

[almaleksia]

When line is out of range GraphQL API doesn't return error and we consider adding PR comment successful though it wasn't posted. The indication that something like this happened is empty thread id. This change adds thread id check and returns error with a hint to LLM how to fix the parameters in the next iteration.

Closes: #1214

[Copilot]

Pull Request Overview

This PR adds error detection and reporting when the GitHub GraphQL API silently fails to add a pull request review comment (indicated by an empty thread ID in the response). This commonly occurs when parameters like line numbers are out of range.

• Added validation to check if the thread ID is nil after mutation
• Returns a descriptive error message with hints for fixing common issues
• Updated existing test mock to return a valid thread ID
• Added new test case to verify the error handling for nil thread ID

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
pkg/github/pullrequests.go	Added nil check for thread ID after addPullRequestReviewThread mutation with helpful error message explaining possible causes (line number out of range, incorrect file path, invalid side)
pkg/github/pullrequests_test.go	Updated existing test mock to return a valid thread ID and added new test case validating error handling when thread ID is nil

[tommaso-moro]

🚀

Love the hint for the LLM! great touch 💅