Improve ShellCheck diagnostic span for WDL interpolations

[kris70lesgo]

Describe the problem or feature in addition to a link to the issues.

ShellCheck diagnostics for WDL command placeholders currently highlight the entire ~{...} placeholder even when only a specific expression inside the interpolation is problematic.
This pr improves diagnostic precision by tracking the span of the interpolated expression and mapping ShellCheck warnings back to that span instead of the full placeholder.
Changes:

• Refactor to_bash_var() to return the span of the interpolated expression.
• Track placeholder -> expression span mapping during command sanitization.
• Update shellcheck_lint() to use the expression span when emitting diagnostics.
  Example

Before:

~{if threads > 1 then "--threads " + (threads - 1) else ""}
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

After:

"--threads " + (threads - 1)
^^^^^^^^^^^^^^^^^^^^^^^^^^^^

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• You have added a few sentences describing the PR here.
• Your code builds clean without any errors or warnings.

For all contributors:

• You have added tests (when appropriate).
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For PRs containing lint rule changes:

• You have updated any and all effected entries within RULES.md.
• You have added a test case in crates/wdl-lint/tests/lints that covers every
  possible diagnostic emitted for the rule within the file where the rule
  is implemented.

[kris70lesgo]

@a-frantz can u review the pr ?

[a-frantz]

this looks like a great start! Can you fix all the issues reported by CI?

[kris70lesgo]

@a-frantz Thanks for the review , all checks are passing now. If u get time can u review it again ?
Thanks