Issue #17727: Add regex pattern to forbid lowercase Javadoc beginnins in google_checks.xml

[guptapratykshh]

Issue: #17727

The default google_checks.xml configuration does not enforce the Google Style Guide rule that Javadoc summary fragments must start with a capital letter. This leads to false negatives where Javadoc summaries beginning with lowercase letters (for example, "adds an element to the list.") are not marked as violations, even though the Google Style Guide states that summaries should be "capitalized and punctuated as if it were a complete sentence."

I added the regex pattern |^\s*[a-z] to the forbiddenSummaryFragments property in the SummaryJavadoc check configuration. This pattern detects Javadoc summaries that start with optional whitespace followed by a lowercase letter, ensuring that lowercase beginnings in Javadoc are marked as violations according to the Google Style Guide.

[guptapratykshh]

@romani, Please look into this PR and review it. Thanks