fix: [serde] Stop flexible_bool inverting non-integer JSON numbers

[elhoim]

BLUF — flexible_bool turned a JSON 0.0 into true; truthiness now goes through as_f64().

• Problem — flexible_bool and flexible_bool_opt in src/models/serde_helpers.rs (:132, :169) tested n.as_i64() != Some(0), but as_i64() returns None for any non-integer number, so a JSON float 0.0 deserialized as true.
• Fix — Both sites decide truthiness with Number::as_f64() != 0.0, keeping fallback-to-true only where as_f64() itself returns None. Adds tests for 0.0, 1.0 and 2.5.
• Effect — Flags such as to_ids, published and deleted can no longer be inverted by malformed or proxied JSON. A compliant MISP emits only 0/1, so this is hardening.
• CI — Red from a known repo CI bug (an ubuntu-22.04 runner with no php8.3 mod_php, plus a publish-worker race in test_event_crud_lifecycle), fixed in PR fix: [ci] Run integration tests on ubuntu-24.04 to match php8.3 #53 and unrelated to this PR.

Finding

n.as_i64() != Some(0) returns true when as_i64() is None
A JSON float 0.0 deserializes to true - a silent inversion rather than an error. Affects :132 and :169.

Change

flexible_bool/flexible_bool_opt in src/models/serde_helpers.rs used n.as_i64() != Some(0) to decide truthiness of a JSON number. serde_json::Number::as_i64() returns None for any non-integer number, so a JSON float like 0.0 produced None != Some(0) = true, silently inverting the value. Fixed both the :132 (flexible_bool) and :169 (flexible_bool_opt) sites to decide truthiness via Number::as_f64() != 0.0 instead, preserving the previous fallback-to-true only for the theoretical case where as_f64() itself returns None (arbitrary-precision feature). Added unit tests for 0.0/1.0/2.5 on both helpers.

Upstream evidence

MISP validates Attribute.to_ids (one of several fields routed through these helpers, alongside published/deleted/distribution flags in event.rs, object.rs, correlation.rs, noticelist.rs, event_report.rs) with CakePHP's boolean rule: MISP core app/Model/MispAttribute.php:427-431 ("'to_ids' => array('boolean' => array('rule' => array('boolean'), 'required' => false)))") @ 843b67445060e4da71ebf75cc4a9646c301f749d. A compliant MISP instance therefore only ever emits 0/1/true/false for these fields, never a float — so this bug is not a wire-protocol mismatch with any specific controller/PyMISP call, it's defensive robustness against malformed or third-party-proxied JSON that happens to send a float. The finding itself is accurate as a pure Rust/serde logic defect (Number::as_i64() returning None for non-integers, inverting the boolean via the != comparison); no additional affected call sites beyond :132/:169 — string_or_i64/string_or_i64_opt already error explicitly on non-integer numbers rather than silently inverting, so they needed no change.

Verified against MISP core 843b6744 and PyMISP e7debb4f.

Tests

• deserialize_flexible_bool_float_numbers
• deserialize_flexible_bool_opt_float_numbers

Caveats

Per task constraints I did not run cargo build/test/clippy (serialized centrally); the two new tests are asserted correct by construction/manual trace (as_i64() returns None for 0.0f64 -> old code's != Some(0) is true; as_f64() returns Some(0.0) for both JSON int 0 and float 0.0 in a standard, non-arbitrary-precision serde_json::Number -> new code returns false), not by an actual cargo test run in this environment. This is a defensive-robustness fix, not a protocol correction — no PyMISP or MISP controller behavior needed to change, and no existing wiremock/integration test asserted the wrong (buggy) 0.0 behavior, so there was no pre-existing wrong test to correct, only new coverage to add.

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Verified locally: cargo fmt --check, cargo clippy --all-targets --all-features -- -D warnings, cargo test --all-features --lib, --test integration_tests --no-run, and cargo check --examples, each in an isolated target directory.

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