fix(function): report code-placeholder compiler invariants as server failures

[waleedlatif1]

Summary

• Two code-placeholder compiler failures are broken invariants in Sim's own compiler, not problems in the user's code: overlapping source edits (applySourceEdits) and exhausting the Python marker attempts (each marker is a hash of the code itself, so user code cannot occupy them)
• They threw CodePlaceholderCompileError, so the Function route answered them with 422 exactly like a user syntax error
• They now throw a sibling CodePlaceholderInvariantError. It deliberately does not extend CodePlaceholderCompileError, so the route's only instanceof check doesn't match it and it falls through to the existing 500 path (the same path other unexpected compiler crashes already take). A subclass or a kind flag would have kept matching that check and stayed 422
• Exhausting the JavaScript parser-sentinel space stays a user compile error: a sentinel is as long as its placeholder, so code under the length cap can contain every sentinel a short placeholder could take
• User placeholder and syntax errors are unchanged (422). No other caller catches these errors by type

Type of Change

• Bug fix

Testing

• Overlapping edits throw CodePlaceholderInvariantError and not CodePlaceholderCompileError (fails on the previous code)
• Code that packs every $xyz$ sentinel before return {{a}} still rejects with CodePlaceholderCompileError
• Compiler and Function route suites, type-check, lint and all audits pass

Checklist

• Code follows project style guidelines
• Self-reviewed my changes
• Tests added/updated and passing
• No new warnings introduced
• I confirm that I have read and agree to the terms outlined in the Contributor License Agreement (CLA)

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1 Skipped Deployment

Project	Deployment	Actions	Updated
docs	Skipped		Sep 29, 2026 4:46am UTC

[cubic-dev-ai]

No issues found across 4 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}

[greptile-apps]

Confidence Score: 5/5

[Medium risk] Changes error classification in code placeholder compilation.

The PR appears safe to merge; no outstanding finding or new actionable defect was identified in its current changeset.

Summary

The PR separates compiler-invariant failures from user compile errors so overlapping edits and Python marker exhaustion follow the Function route’s server-error path. It also tests that user-controlled JavaScript sentinel exhaustion remains a compile error.

Diagram

%%{init: {'theme': 'neutral'}}%%
flowchart LR
  Compiler[Placeholder compiler] --> Error{Error type}
  Error -->|Compile error| Client[Function route: 422]
  Error -->|Invariant error| Server[Function route: 500]

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_{Reviews (3) · Last reviewed commit: "fix(function): keep an exhausted JavaScr..."}

[waleedlatif1]

@greptile

[waleedlatif1]

@cubic-dev-ai review this PR

[cubic-dev-ai]

@cubic-dev-ai review this PR

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[cubic-dev-ai]

No issues found across 4 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}

[waleedlatif1]

@greptile

[waleedlatif1]

@cubic-dev-ai review this PR

[cubic-dev-ai]

@cubic-dev-ai review this PR

@waleedlatif1 I have started the AI code review. It will take a few minutes to complete.

[cubic-dev-ai]

No issues found across 3 files

Confidence score: 5/5

• Automated review surfaced no issues in the provided summaries.
• No files require special attention.

_{Re-trigger cubic}