fix: resolve a contracted registration's dependencies without its contract

[glennawatson]

Summary

A service registered under a contract resolves its dependencies from their registrations without a contract.

• Constructor parameters and injected properties no longer inherit the registration's contract. Register<ViewBase, V1>("V1") now resolves V1's S1 parameter with GetService<S1>(), not GetService<S1>("V1"). The contract still applies to the registration itself, including both registrations of a lazy singleton.
• The README says how a contracted registration's dependencies resolve.

Why

Since the move to an incremental generator, a registration's contract was passed to every dependency it resolved. A contracted service whose dependencies were registered without a contract threw InvalidOperationException: Dependency '...' with contract V1 not registered with Splat resolver. The original generator resolved dependencies without a contract.

Closes #305

Breaking changes

Dependencies registered only under the service's contract no longer resolve.

• A dependency of a contracted registration that was registered only under that same contract no longer resolves. Register it without a contract as well.

How this was verified

• A new test runs registrations the generator wrote into the test assembly against Splat and resolves the issue's scenario. It fails on main with the exception from the issue.
• Snapshots of contracted registrations with dependencies show them resolved without the contract.

Notes for the reviewer

The change is in CodeGeneration/CodeGenerator.cs; the snapshot changes follow from it.

• The generated file now needs only one throw helper, since a missing dependency is never reported under a contract.
• The generator test project now also references the generator as an analyzer, so RuntimeResolutionTests calls the real generated SetupIOC.

Checklist

• I have read the Contribute guide
• The PR title follows Conventional Commits
• Tests cover this change, or the summary says why they do not
• New or changed public API has XML documentation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (9679373) to head (8c4f32a).

Additional details and impacted files

@@            Coverage Diff            @@
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  Coverage   100.00%   100.00%           
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  Files           20        20           
  Lines          976       967    -9     
  Branches       171       168    -3     
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- Hits           976       967    -9     

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