fix(dgw): address reviewer feedback on in-memory credential encryption

[Copilot]

Fixes three issues flagged during review of the in-memory credential encryption PR.

Changes

• Typo fix (rdp_proxy.rs, rd_clean_path.rs): "an the FQDN format" → "in the FQDN format"
• Error classification (credential/mod.rs, preflight.rs): Introduced a typed InsertError enum on CredentialStoreHandle::insert with InvalidToken (client-side bad input) and Internal (server-side failure) variants, so callers can distinguish error kinds without needing to inspect error messages or duplicate logic

pub enum InsertError {
    /// The provided token is invalid (e.g., missing or malformed JTI).
    InvalidToken(anyhow::Error),
    /// An internal error occurred (e.g., encryption failure).
    Internal(anyhow::Error),
}

• Docs (README.md): Document libsodium as a required native build dependency under "From sources", with platform-specific install instructions (Linux/macOS/Windows)

Issue: DGW-326

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[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.

[Benoît Cortier (CBenoit)]

Copilot modify the insert function such that it returns an error type that allows us to distinguish between invalid parameters and internal errors (eg; using an enum). This avoids doing the need for knowing the internals and doing the job twice as precheck (that's an anti pattern)

[Copilot]

Done in commit 1da4403. Added a InsertError enum with InvalidToken and Internal variants to credential/mod.rs. Both CredentialStore::insert and CredentialStoreHandle::insert now return Result<_, InsertError>, with JTI extraction failures mapped to InvalidToken and encryption failures to Internal. The pre-check in preflight.rs is replaced by a match on the error variant.

[Copilot]

Just as a heads up, I was blocked by some firewall rules while working on your feedback. Expand below for details.