Make src gen for property setters consistent with reflection

[steveharter]

Address #86333 (RC2 candidate as well)

• Call the setter for value type properties (with the default value for the property type) if the value is not in the config, unless it is during a Bind (not "Get"). This requires a adding a bool arg to the BindCore() method.
• Always check for a null value or missing section for properties and if null\missing (this was not done in all cases previously) then don't call the setter with the null. This addresses Config generator overwrites existing properties when provided with empty config section #91380.

Note that this PR matches known semantics regarding the above with the goal of maintaining compat with reflection; this is the case even for non-intuitive cases including:

• Only value type properties get their setter called; not reference types or Nullable<T>.
• Also, properties on objects that are nested objects or those added to a collection are also not defaulted through the setter (only properties on root objects are).

We could create an issue for these non-intuitive cases, but would need to consider the benefit:risk including breaking changes.

[ghost]

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Issue Details

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null

Author:	steveharter
Assignees:	steveharter
Labels:	area-Extensions-Configuration
Milestone:	-

[tarekgh]

@steveharter there are some failures that seem related to the change. Could you please have a look?

[layomia]

Thanks!

[steveharter]

Need to get in at least this before backport merge can be successful: #91717

[steveharter]

/backport to release/8.0

[github-actions]

Started backporting to release/8.0: https://github.com/dotnet/runtime/actions/runs/6191249484

[github-actions]

@steveharter backporting to release/8.0 failed, the patch most likely resulted in conflicts:

$ git am --3way --ignore-whitespace --keep-non-patch changes.patch

Applying: Make src gen for property setters consistent with reflection
.git/rebase-apply/patch:218: trailing whitespace.
            Assert.Null(options.OtherCodeUri);            
warning: 1 line adds whitespace errors.
Using index info to reconstruct a base tree...
A	src/libraries/Microsoft.Extensions.Configuration.Binder/gen/Emitter/CoreBindingHelpers.cs
A	src/libraries/Microsoft.Extensions.Configuration.Binder/gen/Specs/Types/SimpleTypeSpec.cs
M	src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.TestClasses.cs
M	src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.cs
Falling back to patching base and 3-way merge...
Auto-merging src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.cs
Auto-merging src/libraries/Microsoft.Extensions.Configuration.Binder/tests/Common/ConfigurationBinderTests.TestClasses.cs
Auto-merging src/libraries/Microsoft.Extensions.Configuration.Binder/gen/Model/ParsableFromStringSpec.cs
Auto-merging src/libraries/Microsoft.Extensions.Configuration.Binder/gen/Helpers/Emitter/CoreBindingHelpers.cs
CONFLICT (content): Merge conflict in src/libraries/Microsoft.Extensions.Configuration.Binder/gen/Helpers/Emitter/CoreBindingHelpers.cs
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0001 Make src gen for property setters consistent with reflection
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!

[github-actions]

@steveharter an error occurred while backporting to release/8.0, please check the run log for details!

Error: git am failed, most likely due to a merge conflict.

[ericstj]

/backport to release/8.0

[github-actions]

Started backporting to release/8.0: https://github.com/dotnet/runtime/actions/runs/6204180109