Reject unsafe unquoted integer floats

[fallintoplace]

Summary

• reject high-magnitude unquoted decimal/exponent JSON numbers once they arrive as integer-valued f64s
• keep exact quoted decimal/exponent strings working for full i64/u64 values
• add regression coverage for the rounded 9007199254740993.0 and exponent forms, including negative i64 cases

Notes

visit_f64 does not expose the original JSON number text, so the parser may already have rounded before the integer visitor runs. The guard keeps small integer-valued float notation working while rejecting larger values that should be quoted for exact parsing.

Tests

• cargo test -p buffa --all-features json_helpers::tests
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace --all-features

[github-actions]

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