[Wei Hu (live1206)]

Summary

Fixes #12304 (the Network constructor follow-up), split out of #12310.

• Preserve a historical concrete-model constructor when the current initialization constructor adds an inherited framework value argument.
• Chain the legacy overload to the current constructor using a typed default only after explicit downstream approval. The capability defaults to false and cannot bypass required source/model-owned properties or ambiguous-target rejection.
• Copy historical parameter metadata without mutating the baseline; reuse the current constructor's initialization and argument validation.

Independence and scope

Design status: constructor/defaulting policy is still under discussion. This draft isolates the existing proposal for review; it is not a finalized decision about how current handwritten constructors should control generation.

This PR targets main independently of #12310 and does not include its mapped-hierarchy/property-projection changes. The constructor fixture supplies an already-restored flat hierarchy; actual flat-base discovery remains #12303.

The downstream management generator must opt into the known historical default alongside Azure/azure-sdk-for-net#63998 before the real Network cleanup uses this path. No real Network GA ApiCompat run, SDK regeneration, or inherited ARM-envelope JSON-roundtrip validation is claimed.

Change-specific validation

• Reproduce the missing properties-only overload on the unchanged generator with the same portable fixture.
• Compile and execute generated legacy/current overloads to check default/explicit Location, Properties identity, exactly-once base initialization, and null-argument validation.
• Verify opt-out, required wire/model-owned property rejection, multiple initialization-target rejection, and unchanged baseline metadata.
• On unchanged production code, the same fixture produced one expected missing-overload failure and five passing controls; after the isolated change, all six Network cases pass.
• After the split: 114/114 focused Network/back-compat checks, 2,923/2,923 core Generator tests, and 2,455/2,455 ClientModel tests passed, with no skipped tests.

- by copilot

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[Wei Hu (live1206)]

Withdrawing this framework-defaulting proposal in favor of the revised plan: keep the known SDK legacy constructor and use existing CodeGenSuppress for explicit replacement, then investigate a narrow custom-constructor restoration-guard relaxation in #12310 if it remains small. That relaxation is not implemented yet; the flat-base and real-SDK validation prerequisites still apply.

The updated cleanup handoff is Azure/azure-sdk-for-net#63992 (comment).

- by copilot