feat(ramps): publish and enforce per-asset buy limits (TRAM-3994)

[wenfix]

Explanation

The v2 ramps API published limits keyed by fiat currency + payment method only, so the extension displayed a single token-agnostic minimum for providers whose minimum varies per token. For Coinbase this showed "Minimum purchase is 2 EUR" regardless of token, even though 2 EUR only holds for ETH and most other tokens require 5 EUR — users only discovered the real minimum on Coinbase's website after redirect, when Confirm & Purchase was greyed out (TRAM-3994).

The backend side (onramp-api) now publishes per-asset limits on the v2 providers response as limits.assets[assetId]. This PR teaches @metamask/ramps-controller to carry and resolve them:

• ProviderLimits gains an optional assets map (ProviderAssetLimitsMap) keyed by CAIP-19 asset id, mirroring the API wire shape, with ProviderAssetLimits and ProviderAssetPaymentLimit for the optional per-payment-method breakdown.
• New getProviderBuyLimit helper resolves a provider's effective buy limit for a fiat currency, payment method, and deposit asset, intersecting the token-agnostic fiat limits with the per-asset limits (tightest bounds win, fiat fees win, maxAmount: 0 treated as unbounded) the same way the ramps API enforces buy limits server-side. Payment method ids match in both bare and /payments/-prefixed form; EVM asset ids match case-insensitively.
• The widened all-providers getQuotes quote pick now applies this helper for buy quotes, so a provider whose limits for the requested asset do not fit the amount is skipped instead of being judged by the token-agnostic fiat minimum. Sell quotes keep the prior fiat-map check, since per-asset limits are a buy dimension.

Note: the client intersects for every provider that publishes asset limits, which can be stricter than API quotes for providers the backend does not enforce per-crypto limits for (its ENFORCE_CRYPTO_PAYMENT_LIMITS_PROVIDERS gate is deployment config invisible to clients). Stricter is the safe direction — it matches what the provider's own checkout enforces.

References

• TRAM-3994 ([Bug]: Ramps - Minimum purchase Coinbase incorrect)
• Root-cause analysis: TRAM-3994 (backend item 1 and extension item 3 are handled separately)
• Backend wire format: onramp-api commit 60623e4a ("fix: publish per-asset limits on v2 providers response")

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've communicated my changes to consumers by updating changelogs for packages I've changed
• I've introduced breaking changes in this PR and have prepared draft pull requests for clients and consumer packages to resolve them