Add cow solver add to the test CLI

[kaze-cow]

Drives the new AddSolver instruction (#119) from the dev CLI:

cow solver add <address> [--manager <keypair-path>]

• New test-cli/src/cmd/solver.rs: a solver command group whose add subcommand builds cow_settlement_client::instructions::AddSolver against the derived state PDA, sends it, and prints a summary (signature, solver, manager, statePda).
• AddSolver needs both the manager's and the payer's signature, so --manager takes a keypair path, not an address. It defaults to by the same as --keypair, which is the payer.

Verification

I deployed a testing program for this base branch for the change at 7hwEhpgqZy1qRNMdFvVh4YjerBrZnMR7qGZUbLJTFSxg

To test adding a solver:

➜  solana-programs git:(kaze/sc-299-add-functions-to-addremove-solvers) ./target/debug/cow solver add B6acm3swJK9pJ7fe4i4GQgP7x5A3RndvsdV2bKhcA1i5 --program-id 7hwEhpgqZy1qRNMdFvVh4YjerBrZnMR7qGZUbLJTFSxg
signature = 2dsTvWAGvxL2w9MVWTHiMn76oKM73XH29GWh5eaKAGfh739A3CrB4ZRhh1hF72HppJdJyRHSmCTUY1gKi6DzwcXE
   solver = B6acm3swJK9pJ7fe4i4GQgP7x5A3RndvsdV2bKhcA1i5
  manager = B6acm3swJK9pJ7fe4i4GQgP7x5A3RndvsdV2bKhcA1i5
 statePda = HkqW1CPzMddJzZm5eFfFBK3wH6uUasLLQ5TczrM2o5VQ

The specified solver account can now be used for a settlement as in #98

🤖 Generated with Claude Code

[linear-code]

SC-299

[fedgiac]

Nice! Approving in advance since the two suggestions are minor and likely uncontroversial (fine if you drop them).