refactor(config): remove redundant wallet field from WalletConfigInner

[Jennycj]

Description

Removes the redundant wallet field from WalletConfigInner. The wallet name was being stored twice: once as the key in WalletConfig::wallets, and once inside each entry as wallet: String. The wallets command's JSON/TOML output showed both.
For example:

{
"my_wallet": {
"wallet": "my_wallet",
"network": "testnet",
...
}
}

The "wallet" field inside the entry is gone and the name is still available as the map key.

Fixes #332

Notes to the reviewers

• TryFrom<&WalletConfigInner> for WalletOpts needed to be removed because it only receives the struct. It is now replaced with a private try_to_wallet_opts(&self, wallet_name: &str) that takes the name from the caller instead.

• Backward compatible: old config.toml files with the stray wallet = "..." line still load (serde ignores unknown fields). This is covered bytest_load_tolerates_legacy_wallet_field.

• CHANGELOG updated under [Unreleased].

Checklists

All Submissions:

• I've signed all my commits
• I followed the contribution guidelines
• I ran cargo fmt and cargo clippy before committing

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.54%. Comparing base (eadbdc7) to head (553a3eb).
⚠️ Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #350      +/-   ##
==========================================
+ Coverage   61.43%   61.54%   +0.11%     
==========================================
  Files          23       23              
  Lines        3993     4005      +12     
==========================================
+ Hits         2453     2465      +12     
  Misses       1540     1540              

Flag	Coverage Δ
rust	61.54% <100.00%> (+0.11%)	⬆️

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