feat(server)!: introduce typed ServerError on the public API boundary

[Greg Lamberson (glamberson)]

Summary

First in a staged migration toward a typed public error story for ironrdp-server, addressing #1209. Introduces:

pub type ServerError = ironrdp_error::Error<ServerErrorKind>;
pub type ServerResult<T> = Result<T, ServerError>;

ServerErrorKind is a #[non_exhaustive] enum with concretely typed variants (Encode, Decode, Io, Channel, Unsupported, Reason, General, Custom). Sources are attached through ironrdp_error::Error::with_source rather than embedded as Box<dyn Error> in variant data. This is byte-for-byte the shape of ConnectorErrorKind in ironrdp-connector, so the connection-management layer stays internally consistent.

Why this shape

The audit before drafting found that ironrdp-connector, ironrdp-session, ironrdp-pdu, ironrdp-mstsgu, and ironrdp-core (EncodeError/DecodeError) all use the same ironrdp_error::Error<Kind> pattern. The thiserror-based bare enums elsewhere in the workspace (GccError, BulkError, etc.) are leaf-layer errors at the PDU/codec level; the connection-management layer sister crate to ironrdp-server is the right template.

Scope of this PR

Public function signatures only:

• RdpServer::run → ServerResult<()>
• RdpServer::run_connection<S> → ServerResult<()>
• TlsIdentityCtx::init_from_paths → ServerResult<Self>
• TlsIdentityCtx::make_acceptor → ServerResult<TlsAcceptor>
• EchoServerHandle::send_request → ServerResult<()>

Internal call sites continue to use anyhow::Result. A private from_anyhow helper bridges at the public boundary. The ConnectionHandler::on_disconnected(error: Option<&anyhow::Error>) parameter is unchanged in this PR.

Companion ext traits

pub trait ServerErrorExt {
    fn encode(error: EncodeError) -> Self;
    fn decode(error: DecodeError) -> Self;
    fn io(context: &'static str, error: io::Error) -> Self;
    fn channel(context: &'static str) -> Self;
    fn unsupported(context: &'static str) -> Self;
    fn general(context: &'static str) -> Self;
    fn reason(context: &'static str, reason: impl Into<String>) -> Self;
    fn custom<E>(context: &'static str, error: E) -> Self
    where E: core::error::Error + Sync + Send + 'static;
}
pub trait ServerResultExt {
    fn with_context(self, context: &'static str) -> Self;
    fn with_source<E>(self, source: E) -> Self
    where E: core::error::Error + Sync + Send + 'static;
}

Mirrors ConnectorErrorExt / ConnectorResultExt exactly.

Staged follow-ups (separate PRs)

• PR Add fuzzers #2: Convert internal ? chains and .context() calls in ironrdp-server to use the typed kinds directly. Remove the anyhow dependency when the last anyhow! is gone. Touches encoder/mod.rs (heaviest), server.rs, display.rs, builder.rs.
• PR Panic in UserDataHeader::from_buffer due to integer overflow #3: Change ConnectionHandler::on_disconnected(error: Option<&anyhow::Error>) to Option<&ServerError>. Standalone breaking change for handler implementations.

Breaking change

Marked with ! in the conventional commit. Consumers of the five listed public functions need to update their Result types. Pre-1.0, and per Marc-Andre Lureau (@elmarco)'s note on #1209: "I am ok with breaking API at this point :)".

Test plan

• cargo xtask check fmt -v clean
• cargo xtask check lints -v clean (workspace, all-targets, with helper + __bench features)
• cargo xtask check tests -v passes
• cargo build --workspace --all-targets clean (one example consumer in crates/ironrdp/examples/server.rs updated to convert at its own boundary)

Closes part of #1209.