fix(model_prices): correct cache read pricing for Gemini flash latest and preview aliases

[Alphaxiaoteng]

Fixes #38062

Root Cause

Several Gemini 2.5 / Flash aliases in model_prices_and_context_window.json (and its backup) still billed cache reads at 25% of input cost (7.5e-08 / 2.5e-08), which was the legacy Gemini 2.0 rate, instead of the 10% rate (3e-08 / 1e-08) matching Google's official Gemini 2.5 Flash pricing:

• gemini/gemini-flash-latest (7.5e-08 -> 3e-08)
• gemini/gemini-flash-lite-latest (2.5e-08 -> 1e-08)
• gemini/gemini-2.5-flash-preview-09-2025 (7.5e-08 -> 3e-08)
• gemini-2.5-flash-preview-09-2025 (7.5e-08 -> 3e-08)
• gemini/gemini-2.5-flash-lite-preview-06-17 (2.5e-08 -> 1e-08)
• gemini-2.5-flash-lite-preview-06-17 (2.5e-08 -> 1e-08)

Changes

• Updated cache_read_input_token_cost in model_prices_and_context_window.json and litellm/model_prices_and_context_window_backup.json to 10% of input_cost_per_token.
• Added unit test in tests/test_litellm/llms/gemini/test_cost_calculator.py covering all corrected model aliases.

Verification & Test Evidence

• pytest tests/test_litellm/llms/gemini/test_cost_calculator.py -> Passed (19 passed, exit code 0)
• pytest tests/test_litellm/test_model_prices_schema.py -> Passed (21 passed, exit code 0)
• python ci_cd/check_files_match.py -> Passed (exit code 0)
• python ci_cd/generate_model_prices_schema.py --check -> Passed (exit code 0)
• ruff check --config ruff-tests.toml tests/test_litellm/llms/gemini/test_cost_calculator.py -> Passed (exit code 0)

[greptile-apps]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[codspeed-hq]

Merging this PR will not alter performance

✅ 31 untouched benchmarks

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_{Comparing Alphaxiaoteng:fix/gemini-flash-cache-pricing (12aecc4) with litellm_internal_staging (31a6756)}

[devin-ai-integration]

Superseded by the rolling registry PR #38207, which carries these values re-verified against the provider's own pricing page/API (DeepInfra publishes $/token and 262144 context for the Kimi K2.5 and K2.7 Code entries). Maintainers asked for a single open model-pricing PR at a time.