[cdac] Add default (empty-version) contract registration logic

[max-charlamb]

Summary

Extracts the default (empty-version) contract registration logic from #129675 into a standalone change.

This establishes a convention on ContractRegistry for registering a contract implementation that is used as a fallback when the target does not advertise a version for that contract:

• Register("", creator) registers an implementation under the empty-string version.
• In TryGetContract, when the target does not advertise a version for the contract, the registry falls back to the empty-string registration if one is present.
• When the target does advertise a version, an exact-version match is still required, and a missing implementation fails (the fallback does not kick in) — preserving version-skew detection.

Changes

• CachingContractRegistry.cs — implement the empty-version fallback in TryGetContract.
• ContractDescriptorBuilder.cs — TryCreateTarget now accepts optional contractRegistrations (defaulting to CoreCLRContracts.Register) so a test can register a contract implementation.
• ContractRegistrationTests.cs (new) — exercises the production CachingContractRegistry through a real ContractDescriptorTarget (via the public Target.Contracts API), across all standard architectures:
  • no advertised version → falls back to the default ("") registration
  • no advertised version and no default registration → fails
  • advertised version → the versioned registration wins over the default
  • advertised version with no matching implementation → version-skew failure (does not fall back to the default)

Testing

All cdac unit tests pass: Microsoft.Diagnostics.DataContractReader.Tests 2611 passed / 16 skipped / 0 failed (including the 16 new test cases), and DataGenerator.Tests 34/34.

[Copilot]

Pull request overview

This PR introduces a convention in cdac contract registration where an empty-string version ("") represents a default implementation that can be used only when the target does not advertise any version for that contract. The production CachingContractRegistry and the test registries are updated to honor this fallback behavior, and new unit tests are added to validate the rule.

Changes:

• Document the empty-string default registration convention on ContractRegistry.Register.
• Implement default (empty-version) fallback resolution in CachingContractRegistry.TryGetContract when the target does not advertise a contract version.
• Update test contract registries to follow the same rule and add new unit tests covering the fallback and version-skew behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/native/managed/cdac/tests/UnitTests/DefaultContractRegistrationTests.cs	Adds unit tests for empty-version default registration and version-skew behavior (currently targets the test registry).
src/native/managed/cdac/tests/TestInfrastructure/TestPlaceholderTarget.cs	Updates the test contract registry to fall back to "" registration only when no version is advertised.
src/native/managed/cdac/tests/DataGenerator/TestTarget.cs	Simplifies the DataGenerator test registry wiring and makes it honor the "" default resolution behavior.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader/CachingContractRegistry.cs	Implements the production fallback logic: use "" registration only when the target doesn’t advertise a version.
src/native/managed/cdac/Microsoft.Diagnostics.DataContractReader.Abstractions/ContractRegistry.cs	Documents the empty-string fallback convention on Register.

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