Java bytecode front-end: fix main method detection

[tautschnig]

When parsing Java classes with just a class name (without method signature), JBMC was incorrectly using resolve_friendly_method_name which could report 'main method is ambiguous' errors. This fix checks if config.main contains a method signature (has a colon) and handles simple class names separately by looking for the standard main method signature directly.

This aligns the Java front-end with Java's specification, where only public static void main(String[] args) is recognized as a valid program entry point.

Fixes: #759

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.