feat(gemini): add topK generation parameter support

[cyppe]

Summary

• Adds topK as a first-class generation parameter via usingTopK(int $topK), following the same pattern as topP and temperature
• Wires topK into all three Gemini handlers (Text, Stream, Structured) within the generationConfig object
• The Gemini API supports topK natively in generationConfig alongside topP, temperature, and maxOutputTokens

Changes

• src/Concerns/ConfiguresModels.php — Added $topK property and usingTopK() fluent method
• src/Text/Request.php / src/Structured/Request.php — Added $topK constructor parameter and topK() accessor
• src/Text/PendingRequest.php / src/Structured/PendingRequest.php — Pass topK through to Request
• src/Providers/Gemini/Handlers/Text.php, Stream.php, Structured.php — Added 'topK' => $request->topK() to generationConfig
• docs/core-concepts/text-generation.md — Documented usingTopK parameter

Usage

$response = Prism::text()
    ->using(Provider::Gemini, 'gemini-2.0-flash')
    ->withPrompt('Hello')
    ->usingTopK(40)
    ->asText();

Test plan

• Added tests for topK in Gemini Text handler (sends topK, omits when not set, works alongside other params)
• Added test for topK in Gemini Stream handler
• Added test for topK in Gemini Structured handler
• Added unit tests for topK in Text and Structured PendingRequest
• Updated existing Request constructor tests to include topK parameter
• Full test suite passes (1469 tests, 0 failures)
• Laravel Pint code style passes
• PHPStan static analysis passes (0 errors)

🤖 Generated with Claude Code