chore(openapi-codegen): drop spec post-processing workarounds and regenerate client

[jaceksan]

Summary

Upstream gdc-nas shipped DX-346, which cleans two shapes in the OpenAPI spec that previously forced this repo to carry Makefile + jq + Python post-processing around openapi-generator-cli:

1. DeclarativeColumn.name no longer carries the NUL-rejecting regex pattern. The constraint moved to a runtime Bean Validation annotation, so the JSON unicode-zero escape stops leaking into the spec.
2. Springdoc sealed-interface oneOf parents no longer emit a redundant child-to-parent allOf ref, breaking the cycle that crashed openapi-generator-cli v6.6.0 with StackOverflowError in recursiveGetDiscriminator.

This PR:

• Drops the corresponding workarounds in Makefile: the NUL-stripping tr, the jq cycle-break on DashboardCompoundComparisonCondition and DashboardCompoundRangeCondition, and the scripts/postprocess_api_client.py regex repair call.
• Deletes the now-dead scripts/postprocess_api_client.py.
• Repoints make api-client STAGING=1 from demo-cicd (production) to staging.dev-latest.stg11.panther.intgdc.com (actual staging). The previous URL silently regenerated against production and would re-introduce the workarounds.
• Regenerates the API client against staging with the workarounds removed (separate commit). The regenerated client picks up the spec drift: DeclarativeColumn.name loses its regex validation block (constraint now in the field description), and a new DeclarativeIpAllowlistPolicy model is exposed under DeclarativeOrganization.

Test plan

• make api-client STAGING=1 completes with no StackOverflowError from openapi-generator-cli.
• No NUL escapes in the merged spec after regen.
• python -c "import gooddata_api_client; from gooddata_api_client import models" reports import OK.
• Live staging spec verified clean across metadata, scan, automation, export, afm, result.
• CI lint + type-check + unit tests pass on the regenerated client.

JIRA: DX-346
risk: low

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.96%. Comparing base (88138a3) to head (194436f).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1594   +/-   ##
=======================================
  Coverage   78.96%   78.96%           
=======================================
  Files         231      231           
  Lines       15578    15578           
=======================================
  Hits        12301    12301           
  Misses       3277     3277           

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