fix: a heredoc closing marker may carry trailing whitespace (#343)

[livingstaccato]

Fixes #343.

What

The spec puts the delimiter "alone on its own line", and Terraform's scanner ends the heredoc at a line holding the word and nothing else that matters — trailing spaces and tabs included. Both heredoc terminals required the newline to follow the word immediately:

hcl2.loads("a = <<EOF\nbody\nEOF  \nb = 1\n")
# lark.exceptions.UnexpectedToken: Unexpected token Token('STRING_CHARS', ...)

So EOF was not a marker at all. The heredoc ran on, swallowed the rest of the file, and the parse failed with an error pointing somewhere else entirely.

OpenTofu v1.12.5 reads the same file as a = "body\n", b = 1.

Why it matters more than it looks

Trailing whitespace is invisible. It survives copy-paste, it is left behind by editors that do not trim it, and it is exactly the kind of thing nobody inspects — so a file someone has been running through Terraform for months can fail here, with a message that names a line far from the real one.

What is still body text

Only whitespace is permitted after the word. EOFX and EOF x remain content, and OpenTofu agrees: it reads

a = <<EOF
EOF x
body
EOF  

as "EOF x\nbody\n", closing at the last line rather than the first.

The change is \s*(?P=heredoc)\r?\n → \s*(?P=heredoc)[ \t]*\r?\n in both HEREDOC_TEMPLATE and HEREDOC_TEMPLATE_TRIM, with eleven tests covering the forms that close and the two that do not, plus the indented and CRLF cases that already worked.

Merging

This one is independent: it touches hcl2/hcl2.lark and nothing else any other open pull request of mine changes, so it can land in any order.

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This pull request, and the investigation behind it, were produced by an AI assistant (Claude) working on behalf of the author. Please review with that provenance in mind.

[livingstaccato]

Please hold off on merging this one for now — I want to do another review pass over it before it goes in. Opened as a draft for that reason; I will mark it ready and say so here once I am done.

[livingstaccato]

Review pass done, so the hold above no longer applies — this is ready for review now.

Rebased on current main; GitHub reports it mergeable as it stands.

_{🤖 Drafted with Claude Code.}