fix(adk): guard nil tool-call args and coerce to empty object in OpenAI-compatible adapters

[alexdarbyshire]

Issue

• Nil tool-call args serialize as "null"
• Occurs after session reload where reloaded session contains tool calls with no args (omitempty in upstream go-genai drops an empty map)
• oMLX returns a 422, ollama returns a 400 to unexpected "null" string

Fixes #2851

Fix

• Add nil guard clauses to OpenAI compatible adapters, coerce to empty object

Tests

• Add regression test for each adapter

Notes

• Considered abstracting in accordance with STYLE.md. Went with in-line guard for readability
• AI used to write/wrangle code

SAP AI

• added defensively for OpenAI compatibility (they support BYO Model)
• Untested if SAP AI model proxy coerces "null" string to {} as several other providers do

References

• Same fix as Bedrock #2222
• Equivalent guard in adk-go's OpenAI adapter.

[supreme-gg-gg]

Can you refactor this to a shared until in models/base.go since this is used across openai, openai responses, sapaicore, and bedrock

[alexdarbyshire]

Extracted nonNilFunctionCallArgs into models/base.go and updated the four callers in 0efab2f6 :)

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