[WIP] Extract Maven repo interaction logic for JVM ecosystems reuse

[AbhishekBhaskar]

What are you trying to accomplish?

Extracts Maven repository interaction logic from Maven::Package::PackageDetailsFetcher into a new abstract base class Maven::Shared::SharedMavenRepositoryClient, enabling SBT (and other JVM ecosystems) to reuse Maven repository resolution without duplicating code.
SBT resolves dependencies from Maven repositories using the same maven-metadata.xml format, URL conventions, and auth patterns as Maven. Rather than duplicating ~300 lines of repository client logic in the SBT ecosystem, we extract it into a shared abstract class that both Maven and SBT can inherit from.

Changes:
New file: shared_maven_repository_client.rb

• Abstract base class (~340 lines) under Dependabot::Maven::Shared
• Extracted logic: URL construction (dependency_parts, dependency_base_url, dependency_metadata_url, dependency_files_url), metadata fetching (XML + HTML), version extraction, response checking, forbidden URL tracking, released? checks, credential/auth header handling, and per-repository metadata caching
• Defines 3 abstract methods subclasses must implement: dependency, credentials, repositories
• Provides overridable hooks: dependency_parts (for non-standard naming), central_repo_url (for credential-based overrides)

Modified file: package_details_fetcher.rb

• Now inherits from SharedMavenRepositoryClient (was standalone class)
• Reduced from ~470 lines to ~220 lines
• Retains only Maven-specific logic:
  - fetch / releases — package details orchestration
  - released? — delegates to parent via super
  - repositories (override) — assembles credential repos + POM-declared repos
  - central_repo_url (override) — delegates to RepositoriesFinder
  - versions / versions_details_from_xml / versions_details_hash_from_html — version list assembly
  - repository_finder / pom_repository_details / pom — Maven POM helpers

Anything you want to highlight for special attention from reviewers?

Method	What SBT implements
dependency	attr_reader — same as Maven, no custom logic needed
credentials	attr_reader — same as Maven, no custom logic needed
repositories	SBT-specific: assemble repos from resolvers in build.sbt + credential repos via inherited credentials_repository_details

How will you know you've accomplished your goal?

If all the existing unit tests pass without failures.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.