fix: indent #guard_msgs code action to match command indentation

[rdavison]

This PR fixes the #guard_msgs "Update with generated message" code action so that the generated doc comment respects the indentation of the #guard_msgs command. Previously the new doc comment delimiters were placed at column 0 and the #guard_msgs command lost its indentation.

For example, applying the code action to an indented invocation such as

namespace Foo
  #guard_msgs in
  #eval IO.println "a\nb"
end Foo

previously produced (note the column-0 -/ and the de-indented #guard_msgs):

namespace Foo
  /--
info: a
b
-/
#guard_msgs in
  #eval IO.println "a\nb"
end Foo

and now produces:

namespace Foo
  /--
info: a
b
  -/
  #guard_msgs in
  #eval IO.println "a\nb"
end Foo

The replaced range runs from the doc comment up to the #guard_msgs token, which includes the command's leading whitespace, so the generated text now reuses that indentation for the closing -/ delimiter and to re-indent the command. The message body stays flush-left because it is compared verbatim (modulo the whitespace mode), matching the existing convention in tests/elab/mutual_coinduction.lean.

🤖 Generated with Claude Code

[mathlib-lean-pr-testing]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 9820f61873fc94ae9fc94b8e23a02336a8741f09 --onto c47a0c7cf035381a2bcdd4cdf2442782eb4a5214. You can force Mathlib CI using the force-mathlib-ci label. (2026-06-03 20:03:10)

[leanprover-bot]

Reference manual CI status:

• ❗ Reference manual CI will not be attempted unless your PR branches off the nightly-with-manual branch. Try git rebase 9820f61873fc94ae9fc94b8e23a02336a8741f09 --onto 803553a556fd82fa1060efb0c43eda542130cb16. You can force reference manual CI using the force-manual-ci label. (2026-06-03 20:03:12)