Add managed hosting SDK and restructure docs

[Shenglong Li (shenglol)]

Introduces Azure.Deployments.Extensibility.Hosting.Managed, the public managed host for third-party, local, and internal Bicep extensions. It adds assembly-based extension identity, exact single-version dispatch, startup/lifecycle validation, and a GET /ping health check, with accompanying unit tests.

Also simplifies the MagicEightBall sample to a single unversioned handler set, and restructures the documentation (getting started, typed handlers, behaviors, validators, SDK reference, and API contract) with docfx API reference now generated from the Core and Managed assemblies.

[codecov]

Codecov Report

❌ Patch coverage is 83.05085% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.46%. Comparing base (90c8f54) to head (1c1aa35).

Files with missing lines	Patch %	Lines
...g.Managed/Builders/BicepManagedExtensionBuilder.cs	70.00%	6 Missing ⚠️
...ensions/BicepManagedServiceCollectionExtensions.cs	71.42%	6 Missing ⚠️
...Managed/Metadata/BicepExtensionDescriptorReader.cs	73.33%	3 Missing and 1 partial ⚠️
...s.Extensibility.AspNetCore/ExtensionApplication.cs	0.00%	1 Missing ⚠️
...ed/Extensions/BicepManagedApplicationExtensions.cs	94.11%	0 Missing and 1 partial ⚠️
...sting.Managed/Metadata/BicepExtensionDescriptor.cs	83.33%	1 Missing ⚠️
...ged/Validation/ManagedExtensionStartupValidator.cs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #397      +/-   ##
==========================================
+ Coverage   66.41%   69.46%   +3.04%     
==========================================
  Files         118      126       +8     
  Lines        2546     2656     +110     
  Branches      269      277       +8     
==========================================
+ Hits         1691     1845     +154     
+ Misses        780      725      -55     
- Partials       75       86      +11     

Files with missing lines	Coverage Δ
...ty.AspNetCore/Builders/ScalarApiExplorerBuilder.cs	77.77% <100.00%> (+77.77%)	⬆️
...sibility.AspNetCore/Extensions/ScalarExtensions.cs	58.09% <100.00%> (+58.09%)	⬆️
...anaged/Resolution/ExactVersionExtensionResolver.cs	100.00% <100.00%> (ø)
...osting.Managed/Validation/ManagedExtensionState.cs	100.00% <100.00%> (ø)
...s.Extensibility.AspNetCore/ExtensionApplication.cs	58.49% <0.00%> (+3.13%)	⬆️
...ed/Extensions/BicepManagedApplicationExtensions.cs	94.11% <94.11%> (ø)
...sting.Managed/Metadata/BicepExtensionDescriptor.cs	83.33% <83.33%> (ø)
...ged/Validation/ManagedExtensionStartupValidator.cs	87.50% <87.50%> (ø)
...Managed/Metadata/BicepExtensionDescriptorReader.cs	73.33% <73.33%> (ø)
...g.Managed/Builders/BicepManagedExtensionBuilder.cs	70.00% <70.00%> (ø)
... and 1 more

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[Gary Li (gary-x-li)]

This PR normalizes several files CRLF → LF, which inflates the diff (e.g. ScalarApiExplorerBuilder.cs shows 47 lines changed, only 1 real). LF is the right target (.editorconfig says so), but main has mixed endings and no .gitattributes.

Can you add a standalone PR adding .gitattributes (* text=auto) + git add --renormalize ., then rebase this on top so the diff shows only real changes?

[Shenglong Li (shenglol)]

Opened #400 with .gitattributes containing * text=auto and a repository-wide renormalization. Once #400 merges, I will rebase this PR onto the updated main so its diff contains only substantive changes.