Prevent stale keyring entry when overwriting a key.

[fnando]

What

Prevent stale keyring entry when overwriting a key

Why

Close #2492

Known limitations

N/A

[Copilot]

Pull request overview

This PR addresses issue #2492 by ensuring Secure Store (keyring) entries are actually updated when a key is overwritten, preventing the CLI from continuing to reference stale key material after a successful-looking overwrite.

Changes:

• Thread an overwrite: bool flag through secure_store::save_secret into StellarEntry::write.
• Change keyring write behavior to error on existing entries unless overwrite is explicitly allowed, and to always persist the new seed phrase when overwriting.
• Add unit tests covering overwrite and non-overwrite behavior for keyring writes.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 4 comments.

File	Description
cmd/soroban-cli/src/signer/secure_store.rs	Adds overwrite parameter and forwards it to keyring entry writes.
cmd/soroban-cli/src/signer/keyring.rs	Implements overwrite-aware keyring writes and adds tests for the new semantics.
cmd/soroban-cli/src/commands/keys/generate.rs	Passes CLI --overwrite through to Secure Store save path.
cmd/soroban-cli/src/commands/keys/add.rs	Passes CLI --overwrite through to Secure Store save path.