fix(provider): accept null for Blockfrost's nullable protocol parameters (#569 part 1)

[Kshot3000]

Fixes part 1 of #569.

Problem

The Blockfrost OpenAPI spec marks 15 fields of /epochs/latest/parameters as nullable: max_val_size, cost_models, cost_models_raw, price_mem, price_step, max_tx_ex_mem, max_tx_ex_steps, max_block_ex_mem, max_block_ex_steps, collateral_percent, max_collateral_inputs, coins_per_utxo_size, min_fee_ref_script_cost_per_byte, drep_deposit and gov_action_deposit. BlockfrostProtocolParameters declared them with Schema.optional, which rejects null — so a single null from Blockfrost failed the whole getProtocolParameters decode with a ProviderError.

Reproduced first: a parameters payload (the test file's OpenAPI-example fixture) with all 15 fields set to null fails getProtocolParameters on current main.

Fix

Each of the 15 fields is now Schema.optionalWith(X, { nullable: true }), exactly as #569 prescribes. A decoded null flows into transformProtocolParameters, whose existing defaults (?? 0n, || 0, and ?. in costModelFromBlockfrost) already handle it — no transform change was needed.

Deliberately left out

• Part 2 (native scripts: cbor: null / serialised_size: null and rebuilding from /scripts/{hash}/json) — a larger change to the script-fetch flow, better reviewed on its own.
• Part 3 (stale fields) — utxo_cost_per_word and consumed_by_tx are unread and safe to drop, but tx_index has an existing test asserting its exact decode past 2^53 (Blockfrost.test.ts, "decodes tx_index past 2^53 exactly"), so I left that call to the maintainers rather than deleting a deliberate test in a drive-by PR.

Tests

• New test: all 15 nullable fields set to null decode through getProtocolParameters and fall back to the documented defaults (0 / 0n, empty cost models).
• Blockfrost provider test file: 15/15 pass.
• Full package suite: 1679 passed / 75 skipped / 0 failed (test files 101 passed / 1 skipped of 102).
• tsc --noEmit and eslint on the changed files are clean. (Prettier already flags both touched files on unmodified main; I matched the surrounding style instead of reformatting unrelated hunks.)
• A patch changeset is included.

---

Independent builder — not affiliated with the Cardano Foundation, IOG or Intersect.
@Kshot9000 (https://x.com/kshot9000)
ADA: addr1q8hnl6vl5a6k3rw3n5g3jtte696zcl76kfatzv7gpswa9r0dj7fma6klq55y4ffm7tf0em09udnyhuk4ah92pl5x9jpqjae44v