fix: make %ex behavior context-independent

[ppkarwasz]

Historically, throwable pattern converters (%ex, %xEx, etc.) behaved in a context-sensitive way:

• If the preceding formatter’s expansion did not end with whitespace, the converter automatically inserted a space before rendering the exception.

In version 2.25.0, this was changed to insert a newline instead of a space, but the behavior was still dependent on surrounding context.

What this change does

This PR removes the context-dependent behavior altogether and makes %ex expansion fully predictable, while remaining backward-compatible:

• When %ex is added implicitly because alwaysWriteExceptions=true:

  • If the pattern already ends with %n, a plain %ex is appended.
  • Otherwise, %notEmpty{%n%ex} is appended. This ensures exceptions are always clearly separated from the main log message by a newline, without adding extra characters when no exception is present.

• When %ex is explicitly included in the pattern by the user:

  • Its expansion is rendered exactly as written in the pattern.
  • It will never prepend a newline on its own.

Why

• Eliminates confusing context-sensitive behavior.
• Makes output consistent and predictable.
• Preserves legacy expectations by only modifying implicitly added %ex.

Closes #3873