Refactor migration specs

[ITaluone]

Refactors all Fallout.Migrate.Specs step tests to exercise the full Migration pipeline instead of calling steps directly, and applies the AV1600 behavior-focused naming convention.

What changed

• RewriteCsprojsStepSpecs, RewriteCsFilesStepSpecs, BumpDotNetVersionStepSpecs, ScriptRewriterSpecs now write real files to a temp directory, run them through Migration.RunAsync(), and assert on the file content read back, instead of calling RewriteCsprojsStep/RewriteCsFilesStep/BumpDotNetVersionStep/ScriptRewriter directly.
• Renamed all [Fact]/[Theory] methods in RewriteCsFilesStepSpecs, BumpDotNetVersionStepSpecs, and ScriptRewriterSpecs to short, present-tense, snake_case descriptions of observable behavior, matching docs/agents/conventions.md's AV1600 convention already used elsewhere.

Why?

Calling steps in isolation let the specs drift from what the real migration pipeline produces (e.g. hardcoded version strings that don't match ResolveFalloutVersionStep's live resolution.

Verification

• Traced each test's input through the full pipeline manually to confirm expected edit counts/content.
• All Migration.Specs green

Additional things to mention:

Does not refactor the BumpDotNetVersionStepSpecs in favor of #572

From: #519 (comment)