feat(bun): add support for bunfig.toml registry config

[sebdanielsson]

Changes

This PR adds support for reading registry config from bunfig.toml when Renovate detects a Bun lockfile.

Context

• This closes an existing Issue, Closes: bun: support bunfig.toml for custom registry detection #24460

AI assistance disclosure

Did you use AI tools to create any part of this pull request?

Code and unit tests generated using GitHub Copilot in "Plan" and "Agent" mode using the model Claude Opus 4.5 (Preview). Commit messages and PR descriptions are manual.

• No — I did not use AI for this contribution.
• Yes — minimal assistance (e.g., IDE autocomplete, small code completions, grammar fixes).
• Yes — substantive assistance (AI-generated non‑trivial portions of code, tests, or documentation).
• Yes — other (please describe):

Documentation

• I have updated the documentation, or
• No documentation update is required

Not sure if or on what page this addition should be documented.

How I've tested my work

I have verified these changes via:

• Code inspection only
• Newly added/modified unit tests
• No unit tests, but ran on a real repository
• Both unit tests + ran on a real repository

The public repository: N/A

[sebdanielsson]

@RahulGautamSingh thanks for the review! I've applied the suggested changes.

[RahulGautamSingh]

Failing tests

[sebdanielsson]

Failing tests

I don't think these failing tests are related to this change?