fix: support exactOptionalPropertyTypes and narrow duplex type

[YevheniiKotyrlo]

Summary

• Enable exactOptionalPropertyTypes: true in tsconfig so emitted .d.ts files don't expand optional properties with | undefined
• Fix code patterns in create-fetch/index.ts that assign possibly-undefined values to optional properties
• Narrow duplex type from "full" | "half" to "half" per WHATWG Fetch spec

Closes #86

Problem

The Prettify<T> mapped type causes tsup to emit | undefined on all optional properties in the inline-expanded generic default, while the constraint stays as an alias reference. With exactOptionalPropertyTypes: true, the expanded default doesn't satisfy its own constraint, producing TS2344 on betterFetch.

Changes

1. tsconfig.json — enable flag

 "strict": true,
+"exactOptionalPropertyTypes": true,

2. src/create-fetch/index.ts — conditional spreads

With exactOptionalPropertyTypes, assigning undefined to an optional property is an error. Four places need conditional spread:

 let opts = {
   ...options,
-  method: keySchema.method,
-  output: keySchema.output,
-  headers: validatedHeaders,
+  ...(keySchema.method != null ? { method: keySchema.method } : undefined),
+  ...(keySchema.output != null ? { output: keySchema.output } : undefined),
+  ...(validatedHeaders != null ? { headers: validatedHeaders } : undefined),
 };

 return {
   url,
-  options,
+  ...(options != null ? { options } : undefined),
 };

3. src/types.ts — narrow duplex

-duplex?: "full" | "half";
+duplex?: "half";

The WHATWG Fetch spec IDL enum only defines "half". Full-duplex was discussed but never standardized.

Verification

Verified on v1.1.21 (current npm release, which builds cleanly):

Gate	Result
tsc --noEmit	0 errors
pnpm build (tsup)	ESM, CJS, DTS all pass
DTS inspection (no | undefined in BetterFetchOption)	Clean
Consumer test (exactOptionalPropertyTypes + skipLibCheck: false)	0 errors (only pre-existing Timer type leak remains)

Note on CI

CI will fail due to a pre-existing build breakage on main — undefined headers variable in utils.ts (introduced in #65, tracked in #85, fix in #81). This PR does not touch utils.ts.

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