fix(runtime): accept blank context lines in apply_patch hunks

[kevin9327]

Summary

When an agent edits a file with the built-in apply_patch tool and one of its @@ hunks contains a blank line written as a truly empty line (no leading space), the whole patch is rejected with Invalid hunk line (expected space, "+", or "-"): and nothing is written. Models produce this shape often: a blank context line between two statements, or an empty line separating two *** Update File: sections. The tool description tells the model it applies "the Codex patch format", and Codex accepts both.

The Codex reference (codex-rs/apply-patch) handles this in two places, mirrored here:

• A bare empty line inside an update hunk is read as an empty context line (streaming_parser.rs: "The normal parser treats a bare empty line in an update hunk as an empty context line").
• If a hunk's old lines end with such an empty line and the match fails, the match is retried without it, because it stands for the final newline, which is not a separate element after the file is split into lines (file_update.rs).

Non-blank lines without a diff marker are still rejected.

Validation

bun test packages/runtime/tests/tools/built-in/apply-patch.test.ts

Before the fix (new tests added, source unchanged): 8 pass, 1 fail.

packages\runtime\tests\tools\built-in\apply-patch.test.ts:
189 |           } else if (marker === "-") {
190 |             oldLines.push(content);
191 |           } else if (marker === "+") {
192 |             newLines.push(content);
193 |           } else {
194 |             throw new Error(`Invalid hunk line (expected space, "+", or "-"): ${line}`);
                            ^
error: Invalid hunk line (expected space, "+", or "-"): 
      at _parsePatch (packages\runtime\src\tools\built-in\apply-patch.ts:194:23)
      at applyPatch (packages\runtime\src\tools\built-in\apply-patch.ts:41:19)
      at <anonymous> (packages\runtime\tests\tools\built-in\apply-patch.test.ts:159:11)
(fail) apply_patch built-in > treats bare blank lines in update hunks as empty context [2.44ms]

 8 pass
 1 fail
 16 expect() calls
Ran 9 tests across 1 file. [479.00ms]

After the fix: 9 pass, 0 fail.

• still rejects non-blank hunk lines without a diff marker passes both before and after, which shows the parser is not widened beyond bare empty lines. The 7 existing tests are unchanged and pass.
• bun test packages/runtime/tests/tools/built-in/built-in-tools-module.test.ts: 5 pass / 1 fail on both main and this branch. The failure (runs bash commands from the workspace root) is pre-existing locally because that test spawns bash.
• ESLint (--max-warnings 0) on both touched files: clean on main and on this branch.
• tsc --noEmit -p packages/runtime/tsconfig.json: clean on main and on this branch.
• Prettier: both files already have unformatted lines on main (16 and 6); this change adds none.

🤖 Generated with Claude Code