fix(hypercore): accept null venue payloads in predicted_fundings

[mergt]

Fix predicted_fundings deserialization failure on unlisted coins

HttpClient::predicted_fundings fails on every call against mainnet. The endpoint is currently unusable.

Error: [predicted_fundings] body=[["0G",[["BinPerp",{"fundingRate":"0.00005", ...
Caused by: invalid type: null, expected struct PredictedFundingVenue at line 1 column 1998

Cause

The exchange sends null for a venue's payload when the coin is not listed on that venue. predicted_fundings models the payload as a bare PredictedFundingVenue, so the first null aborts the whole response.

The first one lands at byte 1995 — coin AI, venue BybitPerp, the 4th of 232 coins:

["AI", [
  ["BinPerp",   {"fundingRate":"0.0", "nextFundingTime":1788091200000}],
  ["HlPerp",    {"fundingRate":"0.0", "nextFundingTime":1788087600000, "fundingIntervalHours":1}],
  ["BybitPerp", null]
]]

Nulls are not an edge case. Measured against live mainnet:

Coins	232, exactly 3 venues each (696 venue slots)
Null payloads	69 — BybitPerp 45, BinPerp 24. HlPerp never null
fundingIntervalHours	present on 608/627 non-null payloads; values 1, 4, 8

Changes

• Option<PredictedFundingVenue> per venue in the return type. None means the coin is not listed there — real information, and distinct from a zero funding rate. A consumer that collapses the two invents a funding spread against the venues that did report one, so the distinction is worth the signature change.
• PredictedFundingVenue::funding_interval_hours: Option<u32> — sent by the exchange, absent from the struct. Option because 19 of 627 payloads omit it, with skip_serializing_if per CLAUDE.md. (This is a response type, never signed, so it is unrelated to the ActionRequest signing concern in c395da8.)
• Coverage in test_http_undocumented_typed_responses, which is the test CLAUDE.md names for exactly this failure mode but which never included this endpoint. Asserting a null exists means reverting the Option is a compile error, not a silent regression:

  error[E0599]: no method named `is_none` found for reference `&PredictedFundingVenue`
  

• Doc comment and CHANGELOG entries.

Why it shipped broken

The struct arrived in 4757e9d, "align SDK with Hyperliquid API docs" — modelled from documentation, which shows the happy-path shape and does not mention that a venue payload is nullable. The only existing test asserts the request serializes to {"type":"predictedFundings"}; nothing checked the response. This is the gap CLAUDE.md already describes:

A serialization test proves nothing about the endpoint existing... the live audits parse responses as serde_json::Value, so they prove an endpoint answers, not that a modelled response type still matches.

Breaking

The signature change breaks callers that destructure the venue tuple. Scope is small: 3 references in the lib, none in hypecli or examples/, so no cd hypecli && cargo check fallout. Warrants 0.3.0. No version bump in this branch — left to cargo release.

Verification

• cargo fmt — clean
• cargo clippy -p hypersdk --all-targets — no new warnings (the 4 too many arguments are pre-existing)
• cargo test --lib — 158 passed, 0 failed
• Live mainnet after the fix: 232 coins, 696 venue slots, 69 nulls, 608 with fundingIntervalHours

Note: the new assertion makes test_http_undocumented_typed_responses hit live mainnet for one more endpoint, consistent with the nine already there.