fix(cli): fix datetime types compilation in separate -types crate (#16289)

* fix(cli): fix datetime types compilation in types crate

Parameterize coreModulePath in the Rust model generator and copy core
serde helper modules into the CLI-generated types crate so that
#[serde(with = "crate::core::...")] attributes resolve at compile time.

- Add coreModulePath config option to BaseRustConfigSchema
- Add getCoreModulePath() to AbstractRustGeneratorContext
- Replace all hardcoded crate::core:: paths with dynamic paths in
  structUtils, UnionGenerator, UndiscriminatedUnionGenerator, AliasGenerator
- Copy flexible_datetime.rs, base64_bytes.rs, bigint_string.rs,
  number_serializers.rs into types crate src/core/
- Add chrono, base64, num-bigint, ordered-float deps to types crate
- Add chrono/ordered_float re-exports to types crate prelude
- Update Cargo.lock patching to include new deps

Closes FER-11024

* fix: add chrono/num-bigint/ordered-float to CLI SDK deps for Cargo.lock resolution

The types crate lists these as dependencies, but without [[package]]
entries in the shipped Cargo.lock, cargo build --locked would reject
the inconsistent lock file. Adding them to the SDK's own Cargo.toml
ensures they are resolved in the lock file naturally.

---------

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

Author: jsklan

generators/cli/changes/unreleased/fix-types-crate-datetime.yml

@@ -0,0 +1,5 @@
+- summary: |
+    Fix datetime, base64, bigint, and number types failing to compile
+    in the generated -types crate by copying serde helper modules and
+    adding missing dependencies.
+  type: fix

generators/cli/package.json

@@ -37,6 +37,7 @@
     "@fern-api/core-utils": "workspace:*",
     "@fern-api/fs-utils": "workspace:*",
     "@fern-api/generator-cli": "workspace:*",
+    "@fern-api/rust-base": "workspace:*",
     "@fern-api/rust-model": "workspace:*",
     "@fern-fern/ir-sdk": "67.0.0",
     "@types/node": "catalog:",

generators/cli/sdk/Cargo.lock

@@ -57,6 +57,7 @@ rustls = ["reqwest/rustls-tls-native-roots", "tokio-tungstenite/rustls-tls-nativ
 anyhow = "1"
 base64 = "0.22"
 bytes = "1"
+chrono = { version = "0.4", features = ["serde"] }
 clap = { version = "4", features = ["derive", "string", "env"] }
 clap_complete = "4"
 clap_mangen = "0.2"
@@ -65,6 +66,8 @@ dotenvy = "0.15"
 futures-util = "0.3"
 httpdate = "1"
 libc = "0.2"
+num-bigint = { version = "0.4", features = ["serde"] }
+ordered-float = { version = "4.5", features = ["serde"] }
 percent-encoding = "2.3.2"
 reqwest = { version = "0.12", features = ["json", "multipart", "stream"], default-features = false }
 serde = { version = "1", features = ["derive"] }