[bugfix] Toml comment roundtrip fixes

[jhheider]

This is generally a simple bugfix. I'm happy to open an issue for discussion, if you like, but it came up when editing some Cargo.tomls, so I worked up a quick fix.

What

Fixes two TOML comment-fidelity bugs in the round-trip / -i path, found while
migrating some sed-on-Cargo.toml edits to yq.

1. Head comment above [[array-of-tables]] was dropped

Given:

[package]
name = "x"

# the binary
[[bin]]
name = "y"

yq -p toml -o toml '.' file.toml (or any -i edit) dropped # the binary.
Head comments on ordinary [table] headers already survived. The decoder
attaches the comment correctly, but the three encoder loops that emit [[...]]
headers never wrote it (unlike the [table] path). They do now.

2. Inline comment on an array item detached

Given:

default = [
  "a",  # why a
  "b",
]

round-tripped to:

default = [
  "a",

  # why a
  "b",
]

The decoder attached every in-array comment as a head comment on the next
element. A trailing comment on the same line as an item is now detected via the
parser's byte ranges and kept as a line comment on that item. The encoder emits
array-item line comments inline and only inserts a blank line before
head-commented elements, so the existing head-comment array layout is unchanged.

Note

The first commit is two unrelated, pre-existing repairs kept separate from the
comment work (happy to split into their own PR if you'd prefer):

• The go-toml v2.4.2 bump (Bump github.com/pelletier/go-toml/v2 from 2.4.0 to 2.4.2 #2762) renamed the single-line unterminated-string
  error, but the decode-error test still asserted the old message, so
  TestTomlScenarios was already failing on master.
• A stray gofmt fix (a comment line in encoder_toml.go was space-indented).

Tests

Round-trip tests added for both cases in toml_test.go (top-level and nested
arrays of tables, plus the inline array-item comment). go test ./pkg/yqlib/ -run Toml passes.

[Copilot]

Pull request overview

This PR fixes two TOML round-trip (-i) comment-fidelity issues in yq’s TOML encoder/decoder path, improving preservation of TOML comments when parsing and re-emitting documents (notably Cargo.toml-like files).

Changes:

• Preserve head comments immediately preceding [[array-of-tables]] headers by emitting those comments before each [[...]] header during TOML encoding.
• Preserve inline (trailing) comments on array items by detecting same-line comments in the TOML decoder (via raw byte-range inspection) and re-emitting them inline during TOML encoding.
• Update TOML decode-error test expectation to match the go-toml v2.4.2 error message, and add round-trip regression tests for the two comment bugs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
pkg/yqlib/decoder_toml.go	Detect whether an in-array comment is inline vs on a new line, and attach inline comments to the preceding element’s LineComment.
pkg/yqlib/encoder_toml.go	Emit head comments before [[array-of-tables]] headers; emit array-item LineComment inline and avoid inserting blank lines that detach trailing comments.
pkg/yqlib/toml_test.go	Add round-trip regression tests for [[array-of-tables]] head comments and array-item inline comments; update decode-error assertion for go-toml v2.4.2.

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