apollo_l1_gas_price_types: currency pair and rate kind

[asaf-sw]

Adds the currency-pair vocabulary the Chainlink oracle work is built on, as the leaf of that stack. No behavior change: nothing constructs or reads these types yet, and config_schema.json is untouched.

Two artifacts, both pub in apollo_l1_gas_price_types:

CurrencyPair (EthUsd, StrkUsd, EthStrk) names which pair a reading, rate or guard trip belongs to. Without it a stale ETH/USD leg and a stale STRK/USD leg are indistinguishable, since the ETH/STRK rate reads both. EthStrk has no Chainlink feed on Starknet, so it is derived from the two USD pairs. It carries two accessors with distinct consumers:

• labels() returns the snake_case strum name under LABEL_NAME_CURRENCY_PAIR, read by the per-pair guard counters in the oracle metrics PR.
• pair_name() returns the display form (ETH/USD) used in guard error messages and as the per-pair key for the rate bounds in the rate-bounds PR.

RateKind with the marker types EthToFri and StrkToUsd carries the pair a rate quotes as a type parameter rather than a field, so a client cannot be handed another rate's config and a pair with no impl fails to compile rather than at runtime. The rate arithmetic PR is generic over RateKind; the Chainlink client PR instantiates it per marker.

Each artifact carries a [Temporary comment] naming its consumer, to be dropped as the consumers land.

strum with the derive feature was already a dependency, so Cargo.toml is unchanged.

Tests

New crates/apollo_l1_gas_price_types/src/test.rs, the crate's first tests:

• pair_name_is_distinct_per_variant: every pair_name arm is a string literal, so a copy-paste between them compiles and passes every other test in the stack. Distinctness is what catches it.
• labels_yield_snake_case_variant_name: pins the label values the metrics PR will register permutations against.
• enum_iter_and_variant_names_cover_all_variants: pins the EnumIter order and the VariantNames list, so a new pair added later cannot silently skip a consumer that iterates.
• rate_kind_markers_map_to_their_pair: pins each marker's PAIR, since a transposed mapping compiles and passes every other test.

Verification

• ./scripts/rust_fmt.sh: clean.
• RUSTFLAGS="-D warnings" cargo clippy -p apollo_l1_gas_price_types --all-targets --all-features: clean.
• SEED=0 cargo nextest run -p apollo_l1_gas_price_types: 4 tests run, 4 passed.

[reviewable-StarkWare]

This change is 

[asaf-sw]

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This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• apollo_l1_gas_price_types: currency pair and rate kind #14979 👈 (View in Graphite)
• apollo_batcher: bound in-flight view calls so they cannot exhaust the blocking pool #14972 : 1 other dependent PR (#14942 )
• apollo_batcher: read class definitions through the class manager in view calls #14967
• main-v0.14.3

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