Share generic-method inference precedence and source selection between discovery modes #7450

Generic method inference currently implements source ordering and duplicate-source selection separately in the source generator and reflection discovery. Share these decisions so future changes apply to both discovery modes. This is the independently mergeable generic-method portion of #7371.

Scope

• Define one internal policy for generic-method source precedence: Arguments, typed data sources, parameter inference attributes, MethodDataSource, then GenerateGenericTest.
• Share duplicate-source joining and declared-type constraint fallback decisions between both modes.
• Document the rules and cover them with one parity matrix exercised in both modes.
• Preserve existing discovery behavior and keep Roslyn/reflection type extraction in their respective adapters.

Acceptance criteria

Both discovery modes consume the shared policy; the matrix covers ordering, duplicate sources, explicit types, and fallback. Relevant generator snapshots and targeted fixtures pass. Class inference and class/method pairing remain in the sibling class-policy issue.