bump-cask-pr: load modified contents with FromContentLoader

[p-linnane]

brew bump --open-pr fails on any cask whose file begins with a comment, as reported in #23735. While recalculating checksums, bump-cask-pr reloads the modified cask source from a string via the generic Cask::CaskLoader.load. FromContentLoader.try_new only matches content whose first non-whitespace token is cask, so a leading comment makes it fall through to the remaining loaders and the tail of the source is misread as a cask token, producing Error: Cask 'texstudio.savedstate", ] end ' is unavailable.

Use Cask::CaskLoader::FromContentLoader explicitly at that boundary, matching the tmp_cask load a few lines above. That was the last string load in the command still going through the generic loader.

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Claude Code (Opus 5) drafted the implementation and tests; I reviewed the diff, verified the new test fails without the fix and passes with it, and ran brew lgtm + targeted specs.

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[Copilot]

🟢 Approval recommended

The targeted loader change is consistent with adjacent code and the regression test covers the failure path.

Pull request overview

Fixes checksum recalculation for casks whose source begins with a comment.

Changes:

• Explicitly loads modified source with FromContentLoader.
• Adds regression coverage for preserving leading comments while updating version and checksum.

File summaries

File	Description
Library/Homebrew/dev-cmd/bump-cask-pr.rb	Corrects modified-content loading.
Library/Homebrew/test/dev-cmd/bump-cask-pr_spec.rb	Tests the reported failure case.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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[MikeMcQuaid]

Makes sense, thanks!