Clarify new resolver config hook source URL behavior and clean up the resolver provider contract #1373

Summary
Define how hook resolvers use source URLs and remove the requirement for unrelated resolvers to accept a hook specific URL argument.

Context
PR #1326 added sdist_server_url to the shared resolver_provider() signature and to resolver implementations that do not use it. The value is forwarded to the existing get_resolver_provider hook.
The resolver proposal describes hook-sdist and hook-prebuilt as delegating resolution to hooks. A hook may already know its own package source. The intended role of Fromager’s default or command supplied URL sdist_server_url is unclear, while the shared method contract now carries a value unrelated to most resolver types.

Acceptance Criteria

• The expected behavior of default and custom sdist_server_url values is defined for hook-sdist and hook-prebuilt.
• Existing hooks that accept or use sdist_server_url have a clear compatibility contract.
• Resolver implementations that do not use sdist_server_url are no longer required to accept it solely because of the shared method contract.
• Targeted tests cover the agreed hook behavior and confirm other resolver types still work.