feat(ramps): adds a controller method for fetching ramps tokens

[georgeweiler]

Explanation

Adds a getTokens method to the ramps-controller. Currently, tokens are fetched in the mobile app via useRampTokens, which duplicates API logic and doesn't persist data. The new method centralizes token fetching in the controller, caches results, and stores tokens in controller state for persistence. This aligns token fetching with other ramps data (countries, eligibility) and prepares for mobile app integration in a follow-up PR.

References

https://consensyssoftware.atlassian.net/browse/TRAM-3018

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

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Note

Adds centralized token fetching and state in the ramps controller.

• New RampsController.getTokens(region?, action='buy') with request caching; normalizes region for cache keys; uses userRegion when omitted; persists tokens in state
• init() now fetches user region, eligibility, then prefetches tokens; tokens cleared when userRegion changes or eligibility fetch fails
• Extends messenger with RampsService:getTokens; adds RampsToken/TokensResponse types and RampsService#getTokens
• Updates tests extensively for tokens flow, caching, and edge cases; changelog updated

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