fix(fetchers): enforce max_body_size in GitHub issue fetcher

[chaliy]

Motivation

• Close a size-limit bypass where GitHubIssueFetcher consumed GitHub API JSON with unbounded json() and returned full issue and comment bodies, ignoring FetchOptions.max_body_size and FetchResponse.truncated.

Description

• Apply FetchOptions.max_body_size (with a 5 MiB fallback) to the formatted GitHub issue/PR markdown before returning it via the fetcher by truncating the output to the configured cap.
• Add a UTF-8-safe truncation helper truncate_to_max_bytes to avoid splitting multibyte characters when trimming output.
• Populate FetchResponse.size and FetchResponse.truncated to reflect the returned content length and whether truncation occurred.
• Add unit test coverage for the truncation helper (including multibyte boundary behavior) and keep existing formatting/fetch behavior intact.

Testing

• Ran formatter: cargo fmt --all (completed successfully).
• Ran targeted tests: cargo test -p fetchkit github_issue -- --nocapture, and the fetcher tests passed (11 passed; 0 failed).
• All modified-unit tests covering truncation and formatting succeeded.

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Codex Task