Samples: HTTP Step Builder demo

[Skymly]

Relates to

Skymly/DesignPatterns#287

Blocked by

Skymly/DesignPatterns#290

What to build

A runnable sample that builds an HTTP-style request via the Step Builder domain: required Url + Method; optional Header / Body; mutex BearerToken vs BasicAuth. Demonstrates generated {Holder}Builder type-state (incomplete Build uncallable) and at least one diagnostic-oriented scenario (e.g. mutex) documented in comments or a second sketch if practical.

Acceptance criteria

• Sample project compiles against the library APIs from DesignPatterns #290
• Happy path: required Url+Method, optional Header/Body, one auth choice, assemble returns a request-like product
• Comments (or a small second example) explain required-step completeness and mutex intent for readers
• Sample stays in this Samples repo only (no DesignPatterns library code changes)
• README / sample index updated if the Samples repo has an existing catalog pattern

Parent / coordination

Execution lives in DesignPatterns.Samples. Library Spec: Skymly/DesignPatterns#287. Generator gate: Skymly/DesignPatterns#290.