feat(stellar-wallet-snap): use shareable permissions helpers in Stellar snap

[Julink-eth]

Explanation

Replace the Stellar snap’s local origin-permission map wiring and validateOrigin with the shared helpers from @metamask/snap-networks-utils.

• permissions.ts still owns the method lists. The map is built with createOriginPermissions: Portfolio is registered with an empty dapp method set (every dapp call is rejected), and the MetaMask origin keeps the existing keyring methods. Stellar has no local/test origin toggle, and this PR does not add one.
• KeyringHandler calls validateOrigin(origin, method, originPermissions) from the shared package. The local wrapper in requestResponse.ts is removed.
• Existing origin tests now pass the map into the shared helper. The keyring suite mocks validateOrigin on @metamask/snap-networks-utils.
  Permission surface is unchanged.

References

• Related to WPN-1926
• Follows feat: create a shared origin permission method (#193)

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