feat(buy-sol) PR 6: document native SOL buys for solvers and colocated drivers

[squadgazzz]

Description

Stacked on #4986.

Colocated drivers and external solvers learn the wire contract from the driver's openapi.yml and the solver DTOs, and neither covers native SOL buys. The openapi still describes buyTokenAccount as a token account the settlement never creates. For a native buy, buyToken is the System Program ID and buyTokenAccount is a wallet, which the payout creates when it's missing.

Solvers see a native buy as a wSOL buy into the taker's wSOL ATA (#4985). The route has to leave that ATA open, because the settlement closes it after the swap to unwrap the payouts (#4986). A route that unwraps on its own, like Jupiter with wrapAndUnwrapSol: true, closes the ATA first, and then the settlement's CloseAccount reverts. Our Jupiter solver sets wrap_and_unwrap_sol: false, so only external solvers can trip over this.

#4982 rewrites the same buyTokenAccount description, so whichever PR lands second gets one small conflict.

Changes

• The driver's openapi describes native SOL buys in buyToken and buyTokenAccount
• The buyDestination docs in both solver DTOs name the wSOL ATA and require the route to leave it open

How to test

Doc change only.

[github-actions]

Reminder: Please consider backward compatibility when modifying the API specification.
If breaking changes are unavoidable, ensure:

• You explicitly pointed out breaking changes.
• You communicate the changes to affected teams (at least Frontend team and SAFE team).
• You provide proper versioning and migration mechanisms.

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Caused by:

• crates/solana-driver/openapi.yml