fix: stop duplicating block-scalar lines that look like comments

[pujitha24]

Motivation:
kustomize/commands/internal/kustfile/kustomizationfile.go preserves
comments across kustomize edit calls with a hand-written, line-based
scan (parseCommentedFields) rather than a YAML-aware parser. It treats
any line that, after trimming leading spaces, starts with '#' as a
comment to reattach to the next recognized top-level field. That
heuristic doesn't know about YAML block scalars: when a field's value
is a literal/folded block scalar (e.g. transformers: |-) and its
content happens to contain lines starting with '#' at the scalar's
indentation, those lines are literal string content, not comments.
They already appear once in the correctly marshaled scalar value, but
the parser also mis-captures them as a "comment" and reattaches that
copy ahead of the next field, duplicating them. Because the duplicate
copy is itself re-captured as a "comment" on the next parse, repeated
kustomize edit set image invocations double the duplicated lines
each time (1 -> 2 -> 4 -> 8 -> 16 ...), growing the file without
bound.

Approach:
Track block-scalar state while scanning: when a line ends with a block
scalar header (|, |-, |+, >, >-, >+, and their explicit-indentation
variants like |2 or |-2), remember that line's indentation. While
inside the block, any non-blank line indented more than the header
line is treated as opaque scalar content and bypasses the
comment/field-match logic entirely, so '#'-prefixed content lines are
no longer mistaken for comments. The block ends at the first non-blank
line whose indentation drops to the header's level or below, matching
how YAML determines block scalar extent. Blank-line handling is
otherwise unchanged.

Known limitation, disclosed rather than fixed here: a block scalar
containing a genuinely blank interior line can still cause the number
of blank lines before the next field to grow by one on each repeated
write. This is a distinct, pre-existing bug (confirmed present with
this change reverted too, growing at the same linear rate) about
blank-line accounting, not the exponential content-duplication bug
reported in the issue below. Fixing it would require a larger rework
of this file's ad hoc comment-preservation model, so it's left out of
scope for this minimal fix.

Validation:

• go test ./commands/internal/kustfile/... -run TestPreserve -v
  (in the kustomize module): passes, including a new regression test,
  TestPreserveBlockScalarCommentLikeContent, which performs 3 simulated
  kustomize edit set image invocations (fresh NewKustomizationFile /
  Read / Write per iteration, matching RunSetImage's real usage) against
  a kustomization.yaml with a '#'-prefixed line inside a transformers
  block scalar, and asserts the line appears exactly once afterward.
  Confirmed this test fails (line duplicated) on the pre-fix code and
  passes after the fix.
• go build ./... and go test ./... in the kustomize module: all pass.
• Built the kustomize binary and reran the exact reproduction from the
  issue (transformers block scalar with two '#'-prefixed marker lines,
  4 repeated kustomize edit set image calls): marker count stays at 1
  throughout, versus exponential 1/2/4/8/16 before the fix.
• gofmt -l and go vet ./commands/internal/kustfile/...: clean.
• make lint (this module's golangci-lint target, matching this repo's
  CI Lint job for the kustomize module): exits 0 with no findings
  against the changed files.

Report: #6225
Signed-off-by: Pujitha Paladugu 10557236+pujitha24@users.noreply.github.com

Fixes #6225

fix: stop duplicating block-scalar lines that look like comments

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[saitejabandaru-in]

LGTM! This fixes a very annoying and hard-to-debug issue with comment duplication in kustomize edit set image. The approach of tracking when we are inside a literal block scalar and respecting the indentation level to determine scalar boundaries is extremely sound. Thanks for catching this!

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