feat(bitget): wire real signers for Cosmos + Solana (V3 flow)

[towanTG]

Summary

• Cosmos: drop the dead `signTransaction: async () => ({} as any)` stub. Spread → `Object.assign(offlineSigner, { getAddress })` so `signAmino`/`getAccounts` keep their `this` binding when the toolbox calls them.
• Solana: stop spreading the provider — `Object.assign(provider, { getAddress })` so `signTransaction` / `signAndSendTransaction` stay bound to the provider instance the toolbox uses for V3.

Why

The V3 SwapKit plugin invokes `wallet.signAndBroadcastTransaction(payload)`. The Cosmos / Solana toolboxes synthesize that method on top of the signer's primitives. Today:

• BitGet Cosmos hands the toolbox a fake `signTransaction` stub layered on top of the real OfflineAminoSigner — when V3 lands, the stub wins and broadcasts `{}`.
• BitGet Solana spreads the provider, which can lose `this` binding on class-bound methods (Phantom uses the provider directly for the same reason).

Notes

Amino-only signer is sufficient — V3 swaps on Cosmos Hub use `MsgSend` / IBC `MsgTransfer`, both amino-safe. We can swap to dual signer later if a CosmWasm-bound chain comes in.

Test plan

• Connect BitGet → Cosmos, run a V3 ATOM swap, confirm txid on Mintscan
• Connect BitGet → Solana, run a V3 SOL swap, confirm signature on Solscan
• Regression: legacy `wallet.transfer({...})` for both chains still works

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