chore: replace deprecated --allow-extra-fields codegen flag

[XiaolongZhang-TT]

Description

datamodel-code-generator deprecated the --allow-extra-fields flag in favor of --extra-fields=allow. generate_models.sh still passes the deprecated alias, so every run emits:

DeprecationWarning: --allow-extra-fields is deprecated. Use --extra-fields=allow instead.

The alias will be removed upstream, at which point generate_models.sh breaks outright. This swaps to the supported --extra-fields=allow.

Verified byte-identical codegen output between the two flags — diff -r over the full release/2026-04-08 schema set with all other options unchanged turned up no differences, so the committed models are untouched and the #62 regeneration-equality check still passes. Complements the generation-integrity work in #62.

Category (Required)

Please select one or more categories that apply to this change.

• Core Protocol: Changes to the base communication layer, global context, or breaking refactors. (Requires Technical Council approval)
• Governance/Contributing: Updates to GOVERNANCE.md, CONTRIBUTING.md, or CODEOWNERS. (Requires Governance Council approval)
• Capability: New schemas (Discovery, Cart, etc.) or extensions. (Requires Maintainer approval)
• Documentation: Updates to README, or documentations regarding schema or capabilities. (Requires Maintainer approval)
• Infrastructure: CI/CD, Linters, or build scripts. (Requires DevOps Maintainer approval)
• Maintenance: Version bumps, lockfile updates, or minor bug fixes. (Requires DevOps Maintainer approval)
• SDK: Language-specific SDK updates and releases. (Requires DevOps Maintainer approval)
• Samples / Conformance: Maintaining samples and the conformance suite. (Requires Maintainer approval)
• UCP Schema: Changes to the ucp-schema tool (resolver, linter, validator). (Requires Maintainer approval)
• Community Health (.github): Updates to templates, workflows, or org-level configs. (Requires DevOps Maintainer approval)

Related Issues

None.

Checklist

• I have followed the Contributing Guide (including Conventional Commits title requirements and ! for breaking changes).
• I have updated the documentation (if applicable).
• My changes pass all local linting and formatting checks.
• I have added tests that prove my fix is effective or that my feature works. (N/A for a build-script flag; equivalence is verified by the diff in Description and the ci: verify committed models match regeneration from the pinned UCP spec #62 CI.)
• New and existing unit tests pass locally with my changes.
• (For Core/Capability) I have included/updated the relevant JSON schemas.
• I have regenerated Python Pydantic models by running generate_models.sh under python_sdk. (No regeneration needed — output is byte-identical; see Description.)

Screenshots / Logs (if applicable)

The one-line change:

-    --allow-extra-fields \
+    --extra-fields=allow \

Equivalence check (full schema set, only the flag differing):

$ diff -r out_allow_extra_fields out_extra_fields_allow
$ echo $?
0