Fix superfluous comma in flow style mappings

[sidsri14]

Summary

Fixes #110 — a flow-style mapping that carries a comment (e.g. a comment line below a flow mapping sequence item, which retain_line_breaks injects) was emitted with a superfluous trailing comma: - {foo: bar,}.

The flow-mapping MAPPING_END branch in yaml_emitter_emit_flow_mapping_key wrote a separator comma whenever any comment buffer was non-empty (!first && !trail). Comments on/below a flow mapping attach to the mapping node and arrive with the FLOW_MAPPING_END event, so that condition produced a comma before the closing brace even though no separator was needed.

Interior flow-map comments always flow through the TRAIL path (yaml_emitter_emit_flow_mapping_value), so a comma is never legitimately required at MAPPING_END. This change aligns the flow-mapping branch with the flow-sequence END branch, which already writes the comma only in canonical mode.

- if (emitter.canonical || len(emitter.head_comment)+len(emitter.foot_comment)+len(emitter.tail_comment) > 0) && !first && !trail {
+ if emitter.canonical && !first && !trail {

Test plan

Added a regression case to TestFormatter using the exact reporter scenario (include_document_start + retain_line_breaks, blank line below - {foo: bar}):

• Before fix: output is ---\n- {foo: bar,}\n → test fails
• After fix: output is ---\n- {foo: bar}\n → test passes

Verified the full go test ./... failure set is identical with/without the change (no regressions), and gofmt/go vet are clean.

Notes

• This bug is inherited from upstream yaml.v3's emitter logic (identical condition in go.yaml.in/yaml/v3), not introduced by the yamlfmt fork.
• AI assistance: this patch was developed with the aid of an AI coding assistant (Claude Code); root cause, fix, and test were reviewed and verified manually.

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.