feat(sdk-coin-sol): add addFeePayerSignature()

[ralph-bitgo]

What

• Adds addFeePayerSignature(publicKey, signature) to Transaction and TransactionBuilder in modules/sdk-coin-sol.
• The method wraps the existing addSignature() and rejects a key that is not account 0 of the message (the fee payer): for a versioned transaction that is the first static account key; for a legacy transaction @solana/web3.js places the fee payer first. On both message kinds the signature lands in slot 0.

Why

• Sponsored SOL transactions (CHALO-983 PRD, SOL Gasless Fee Payer) are paid by the enterprise fee address, whose HSM single-sig signature must end up in slot 0 next to the wallet signature. SOL has no such method today (APT and SUI do).
• This is the CHALO-996 scope, pulled in as the dependency surface of the CHALO-1652 fee-payer rewrite (part 2 of this stack), because CHALO-996 had not landed when this work started. If CHALO-996 lands first, this PR can be dropped in favor of it.

Test plan

• npx mocha test/unit/transaction.ts: the fee-payer signature fills the empty signature slot of RAW_TX_UNSIGNED and reproduces RAW_TX_SIGNED byte-for-byte; a non-account-0 key and an unbuilt transaction are rejected
• npm run lint and tsc --build clean in modules/sdk-coin-sol
• Full module unit suite green (761 passing)

Ticket: CHALO-1652

Stack

This PR is part 1 of 2 in a stack. Review and merge in order:

1. feat(sdk-coin-sol): add addFeePayerSignature() #9848 — addFeePayerSignature() (CHALO-996 dependency surface; base: master) ← you are here
2. feat(sdk-coin-sol): rewrite fee payer for versioned custom transactions #9849 — fee-payer rewrite for versioned custom transactions (base: CHALO-1652-fee-payer-rewrite-versioned-tx-pt1)

[linear-code]

CHALO-1652

[github-actions]

⚠️ Unit tests are failing on Node 26.x (Current release line, non-blocking). This is not an LTS version yet, so it does not block merge, but it signals an incompatibility to fix before Node 26.x becomes LTS.

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