fix(ssh): repair pasted private key material and negotiate RSA signature hash

[coder-hhx]

Problem

SSH tunnel connections using private-key auth fail with generic errors ("Invalid SSH private key: Could not read key" / "SSH authentication failed") in several common setups.

Root causes

1. Pasted key material breaks russh's strict PEM reader. russh::keys::decode_secret_key matches -----BEGIN/END ...----- markers by exact line equality and silently drops any base64 line containing a non-base64 character. A key pasted into settings with indentation, a trailing space on any line, collapsed newlines (single-line paste), literal \n escapes (copied out of JSON), CRLF endings, or a BOM fails to decode even though it looks fine in the UI. Verified against russh 0.62.2:
   • indented lines → SshKey: length invalid
   • trailing whitespace / single-line → Could not read key
2. RSA signature hash was hardcoded to rsa-sha2-256. Servers that only accept legacy ssh-rsa (old OpenSSH/embedded devices) reject the auth outright.
3. Unhelpful errors for encrypted keys. A missing or wrong passphrase surfaced as "Invalid SSH private key: The key is encrypted" / "SshKey: cryptographic error".

Fix

• normalize_ssh_private_key_material: strips BOM/zero-width chars, normalizes CRLF and literal \n/\r\n escapes, trims per-line whitespace, and canonically re-folds the PEM block at 64 columns. Legacy encrypted PEM (Proc-Type:/DEK-Info: headers) and PuTTY PPK content are intentionally left line-structured. Applied in resolve_ssh_auth_material for both pasted keys and keys read from privateKeyPath, so existing mangled stored keys are repaired at connect time.
• RSA hash is now negotiated from the server's server-sig-algs extension (RFC 8308) via best_supported_rsa_hash(), falling back to the previous SHA-256 behavior when the server does not advertise the extension. Non-RSA keys skip the negotiation (avoids the extension-info wait).
• describe_ssh_private_key_decode_error maps decode failures to actionable messages ("key is encrypted; configure the key passphrase", "Invalid SSH private key or wrong passphrase: ...").

No UI changes (backend-only), so no gateway WebUI mirror is needed. The gateway has no parallel SSH implementation; the Tauri terminal runtime is the single connect path (tunnel + SFTP).

Tests

• 8 new unit tests covering: all paste-artifact variants normalize to the canonical form and decode; normalization idempotency; encrypted-PEM header and PPK passthrough; resolve_ssh_auth_material with pasted and file-based keys; encrypted key with passphrase; error messages for missing/wrong passphrase.
• Full suite: cargo test --lib → 518 passed. cargo clippy --lib --tests introduces no new warnings.