refactor(core): derive Display and Error via thiserror in DecodeErrorKind and EncodeErrorKind

[Greg Lamberson (glamberson)]

Refs #1257. Companion to #1260.

Companion to the std-only thiserror migration in #1260. ironrdp-core is split off because it is no_std-aware and the change requires Cargo.toml feature wiring that the other crates do not need.

Changes

• Adds thiserror = { version = "2", default-features = false } to dependencies.
• Wires thiserror/std into the existing std feature: std = ["alloc", "ironrdp-error/std", "thiserror/std"].
• Derives thiserror::Error on DecodeErrorKind and EncodeErrorKind with per-variant #[error("...")] attributes that match the prior write!(f, "...") format strings exactly.
• Each cfg-gated UnsupportedValue variant (with/without alloc) receives its own #[error("...")] attribute.
• Removes the #[cfg(feature = "std")] impl core::error::Error for *ErrorKind {} gates. core::error::Error lives in core (stabilized Rust 1.81, well below MSRV 1.89), and thiserror's derive emits an unconditional impl. The behaviour change is from "Error impl available only with std" to "Error impl always available", which is a strict superset.

Public API impact

None. The pub type DecodeError = ironrdp_error::Error<DecodeErrorKind> and pub type EncodeError = ironrdp_error::Error<EncodeErrorKind> aliases are unchanged. Consumers see no source-level break.

no_std verification

All three feature combinations build cleanly:

• cargo check -p ironrdp-core --no-default-features (no_std, no alloc)
• cargo check -p ironrdp-core --no-default-features --features alloc (no_std + alloc)
• cargo check -p ironrdp-core --features std

xtask verification

• cargo xtask check fmt: clean
• cargo xtask check lints: clean
• cargo xtask check locks: clean (both root Cargo.lock and fuzz/Cargo.lock updated and included; the missing fuzz lock was the failure mode of the earlier refactor: derive Display and Error via thiserror across core, pdu, connector, session #1259 attempt)
• cargo xtask check typos: clean
• cargo xtask check tests: clean

Diff size

5 files changed, +20 −73 (net −53 lines of code removed).

[Greg Lamberson (glamberson)]

Closing per #1257 (#1257 (comment)). Change 1's premise is withdrawn after the Core Tier proc-macro invariant correction.