fix: parse string-encoded organization costs

[patrickswedish]

Summary

• accept numeric strings returned by the organization costs API when reading amount.value
• keep the SDK's global JSON coercion rules and raw-field representation unchanged
• continue rejecting malformed and non-finite string values

Why

The costs endpoint can return monetary values as strings, including scientific notation such as "0E-6176" and high-precision decimals. Because the SDK intentionally disables global string-to-float coercion, those values remain raw JsonString instances and Amount.value() throws OpenAIInvalidDataException.

This change handles the wire-format variation only at the affected accessor. Normal JSON numbers, missing/null fields, and invalid values retain their existing behavior.

Tests

• added focused regression coverage for scientific-notation and long-decimal strings
• added negative coverage for malformed numeric strings
• ./gradlew :openai-java-core:lint
• git diff --check

Fixes #769

[chatgpt-codex-connector]

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[jbeckwith-oai]

Reviewed exact head 07dfe0fc5b88597f160243dfbe560ddd40c0647a comprehensively. The fallback is correctly scoped to the affected organization-cost accessor, so it leaves global Jackson coercion, the public Optional<Double> API, builders, raw JsonField representation, and serialization unchanged. The regex implements JSON's decimal/scientific number grammar; conversion preserves the SDK's existing Double precision contract, accepts the reported underflow/high-precision payloads, and rejects malformed or non-finite/overflowing string values. Existing numeric values and missing/null fields retain their prior paths and behavior.

Validation: focused UsageCostsResponseTest passed under JDK 21; :openai-java-core:lint passed; diff check passed; and the branch merges cleanly with current main. The earlier automated finding about JVM-only numeric spellings is addressed by the exact-head grammar guard and negative test matrix. No blocking findings. (GitHub has not published remote checks for this external-contributor head.)