fix: discard leading comment/whitespace preamble in parseYaml stream

[ricardbejarano]

Summary

Fixes #660.

When a YAML document stream begins with comment lines or blank lines before the first --- document-start marker, std.parseYaml incorrectly emits a spurious null as the first element of the result array.

Root cause: YAMLReader.read() accumulates all input lines into a buffer until it encounters a --- marker. At that point, if the buffer is non-empty it returns the buffered bytes as a document and resets. When those bytes contain only YAML comments (#...) or blank lines, yaml.Unmarshal yields nil, which is then appended as null to the result.

Fix: Introduce isCommentOrWhitespace to detect when the accumulated buffer contains only comments and blank lines (valid YAML preamble per §9.2 of the YAML spec). When that is the case, discard the buffer rather than returning it as a document.

Before:

std.parseYaml("# Test\n---\nfoo: bar\n---\nbaz: cuux\n")
// => [null, {"foo": "bar"}, {"baz": "cuux"}]

After:

std.parseYaml("# Test\n---\nfoo: bar\n---\nbaz: cuux\n")
// => [{"foo": "bar"}, {"baz": "cuux"}]

Changes

• yaml.go: add isCommentOrWhitespace helper; skip preamble-only buffer in YAMLReader.read()
• testdata/parseYaml.jsonnet: add two regression test cases (single leading comment; multiple comments + blank lines)
• testdata/parseYaml.golden: update expected output accordingly

All existing tests continue to pass (go test ./...).

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