fix: heredocs_to_strings writes a value, not the heredoc's text (#337)

[livingstaccato]

Fixes #337.

What

The option converts a heredoc into a quoted string, and was quoting the heredoc's own source — markers and all — across as many physical lines as the original occupied:

a = "<<EOT
hello
EOT"

A quoted template cannot span lines. OpenTofu rejects that with "Invalid multi-line string", and reading it back here gives '<<EOT\nhello\nEOT' — the marker text rather than the value. So the option produced neither a valid file nor the right content, and it is exposed on the shipped jsontohcl2 script, so the CLI had the same hole.

After:

a = "hello"

How

By reusing the flattening the reader already performs, rather than writing a second one: serializing the rule with preserve_heredocs=False is exactly the quoted form that option produces on the way in. The two therefore cannot drift apart the way the escape alphabets did in #329 — there is only one implementation of "what is this heredoc as a quoted string".

What that inherits

The value carried is the reader's. On main that still drops the body's trailing newline, so <<EOT\nhello\nEOT becomes "hello" where Terraform reads "hello\n". That is #326, fixed in #335, and this change deliberately follows whatever the reader does rather than second-guessing it — when #335 lands, this output gains the newline with no change here.

Eight tests, including that every converted form parses again and that no marker survives into the output.

Merging

It touches the same code as #346 (hcl2/deserializer.py), #354 (hcl2/deserializer.py). Whichever of those lands first, this one needs a rebase rather than a merge — the overlaps are real edits to the same methods, not adjacent lines, so resolving them by hand risks losing one of the two fixes. Say the word and I will rebase and re-run the suite.

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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. Every reproduction, test run and benchmark cited was executed rather than inferred, but 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.