fix: detect Quarto engines from file content when quarto inspect is u…

[zackverham]

Summary

Fixes #3993 — Quarto documents with R code chunks (e.g. source("script.R")) were generating an empty engines field in the manifest, causing Connect to skip renv restore.

Root cause: Two gaps in engine detection:

1. When quarto inspect fails (quarto not in the extension host's PATH), the fallback path handled .ipynb → engines: ["jupyter"] and .Rmd → engines: ["knitr"], but for .qmd files it never read the file content to detect {r} or {python} code chunks — leaving engines empty.

2. When quarto inspect succeeds on a directory but returns an empty engines array (edge case with certain quarto configurations), the code skipped file-content language detection entirely.

Fix: In both paths, scan .qmd file content using the existing detectMarkdownLanguagesInContent helper to identify code chunks and populate engines accordingly.

Test plan

• New test: fallback .qmd with {r} chunks → engines: ["knitr"]
• New test: fallback .qmd with {python} chunks → engines: ["jupyter"]
• New test: directory inspection with empty engines falls back to file scanning
• All 37 quarto detector tests pass
• All 450 inspect + bundler tests pass
• Manual: publish a .qmd with source("script.R") from Positron without quarto in PATH — verify TOML config gets engines = ["knitr"]

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Code Review Complete

• Read changed files and understand implementation
• Review code for correctness, bugs, and edge cases
• Check test coverage
• Provide detailed feedback

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Summary

This PR fixes issue #3993 where quarto inspect failures (e.g., quarto not installed) or empty engine results caused the fallback path to not detect R/Python code chunks from .qmd files, resulting in empty engines and skipped renv restore on Connect.

Changes:

1. Enhanced genNonInspectConfig() to scan .qmd file content for {r} and {python} code chunks
2. Added fallback in configFromInspect() for when quarto inspect succeeds but returns empty engines

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Review: ✅ Approved

The implementation is well-structured and addresses the issue correctly.

What's good:

• Uses the existing detectMarkdownLanguagesInContent() helper for consistency
• Proper early-exit optimization when both languages are detected
• Engines are sorted for deterministic output
• Error handling with silent fallbacks for unreadable files
• Tests reference the issue number (Quarto document with sourced R file generates empty engines field in manifest, causing Connect to skip renv restore #3993) for traceability

Code correctness verified:

• quarto.ts:543 - The condition if (!needR || !needPython) correctly continues scanning until both languages are detected
• quarto.ts:187 - The if (needR && needPython) break; correctly short-circuits the loop
• quarto.ts:565-568 - Engines array only updates cfg.quarto when non-empty

Test coverage:

• ✅ Fallback with R chunks detects knitr engine (lines 351-371)
• ✅ Fallback with Python chunks detects jupyter engine (lines 373-389)
• ✅ Directory inspection with empty engines falls back to file scanning (lines 429-448)

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Minor Suggestions (non-blocking)

1. PR template is incomplete - The checkboxes for Type of Change, Approach, User Impact, etc. are all unchecked. Consider filling these out for better documentation.

2. CHANGELOG entry - The PR checklist indicates no CHANGELOG update. Given this is a user-impacting bug fix, consider adding an entry under "Fixed" referencing issue Quarto document with sourced R file generates empty engines field in manifest, causing Connect to skip renv restore #3993.

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Verdict: This is a solid, well-tested fix. The implementation correctly addresses both scenarios: (1) when quarto inspect fails entirely, and (2) when it succeeds but returns empty engines. Ship it! 🚢