Extract provider and token modules into codex-auth

Move the foundational provider and token modules into codex-auth while keeping codex-core as the facade. Also move the corresponding unit tests and record the 3-PR migration checkpoints.

Co-authored-by: Codex <noreply@openai.com>

codex-rs/codex-auth/Cargo.toml

@@ -0,0 +1,24 @@
+[package]
+name = "codex-auth"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lints]
+workspace = true
+
+[dependencies]
+base64 = { workspace = true }
+chrono = { workspace = true, features = ["serde"] }
+codex-api = { workspace = true }
+codex-app-server-protocol = { workspace = true }
+http = { workspace = true }
+schemars = { workspace = true }
+serde = { workspace = true, features = ["derive"] }
+serde_json = { workspace = true }
+thiserror = { workspace = true }
+
+[dev-dependencies]
+maplit = { workspace = true }
+pretty_assertions = { workspace = true }
+toml = { workspace = true }