Strengthen Config v2 conversion boundaries

[MrAlias]

Summary

Internal Config v2 hardening work split from the CLI surface (#2679 and #2682) so the conversion contract can be reviewed independently.

• Reject selectors and declarative fields that the legacy runtime cannot represent.
• Preserve selector semantics while applying Config v2 defaults.
• Separate static configuration validation from host-dependent checks for standalone and receiver callers.
• Harden schema parsing and generated-document handling.

Verification

• make generate/all
• go test ./internal/config/convert ./internal/config/schema ./pkg/obi

[codecov]

Codecov Report

❌ Patch coverage is 85.87361% with 38 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.36%. Comparing base (bc8036a) to head (94b230a).

Files with missing lines	Patch %	Lines
internal/config/schema/schema.go	72.50%	17 Missing and 16 partials ⚠️
pkg/obi/config.go	92.68%	2 Missing and 1 partial ⚠️
internal/config/convert/export.go	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2700      +/-   ##
==========================================
+ Coverage   69.70%   70.36%   +0.65%     
==========================================
  Files         354      354              
  Lines       49726    49975     +249     
==========================================
+ Hits        34663    35163     +500     
+ Misses      12897    12632     -265     
- Partials     2166     2180      +14     

Flag	Coverage Δ
integration-test	50.59% <47.36%> (-0.34%)	⬇️
integration-test-arm	25.69% <48.78%> (-0.86%)	⬇️
integration-test-vm-5.15-lts	?
integration-test-vm-6.18-lts	?
k8s-integration-test	29.61% <51.21%> (-4.98%)	⬇️
oats-test	31.22% <48.78%> (-3.81%)	⬇️
unittests	65.56% <85.71%> (+0.10%)	⬆️

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[mariomac]

👍

[grcevski]

LGTM!