fix(diagnostics): prevent panics in indented multiline frames

[edjubert]

What kind of change does this PR introduce?

Bug fix.

What is the current behavior?

Rendering a multiline syntax diagnostic can panic when its span ends at column zero
of an indented line.

This happens with a WITH statement followed by a blank line and an indented CTE.

Fixes #785.

What is the new behavior?

Both diagnostic frame renderers clamp the last-line marker start to the span end
before constructing the range.

The CLI now reports the syntax diagnostic normally instead of panicking.

Additional context

Regression coverage includes:

• generic diagnostic frame rendering
• the original CLI reproduction with an indented CTE after a blank line

Validation:

• cargo test -p pgls_diagnostics
• cargo test -p pgls_cli check_indented_cte_after_blank_line_does_not_panic
• cargo fmt --all -- --check
• cargo clippy --all-targets --all-features -- -D warnings