fix(image_edit): add drop_params support and fix Vertex AI config

[castrapel]

Relevant issues

Solves three related issues in the image edit API for Vertex AI (including #17719 ):

1. drop_params not respected - The image edit API was not honoring litellm.drop_params or the explicit drop_params parameter, this led to UnsupportedParamsError even when users configured silent dropping of unsupported parameters.
2. vertex_project/vertex_location not read from litellm_params - The Vertex AI Gemini image edit config was not checking litellm_params for project/location values, only falling back to environment variables.
3. Incorrect URL for vertex_location: "global" - When using the global location, the URL was incorrectly created as https://global-aiplatform.googleapis.com instead of https://aiplatform.googleapis.com.

Pre-Submission checklist

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• I have Added testing in the tests/litellm/ directory, Adding at least 1 test is a hard requirement - see details
• My PR passes all unit tests on make test-unit
  973 passed, 4 failed (pre-existing upstream failures unrelated to this PR, some appear to be due to my lack of AWS credentials)
• My PR's scope is as isolated as possible, it only solves 1 specific problem

CI (LiteLLM team)

CI status guideline:

• 50-55 passing tests: main is stable with minor issues.
• 45-49 passing tests: acceptable but needs attention
• <= 40 passing tests: unstable; be careful with your merges and assess the risk.

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Type

🐛 Bug Fix

Changes

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Project	Deployment	Review	Updated (UTC)
litellm	Ready	Preview, Comment	Dec 16, 2025 9:31pm

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