[snuff12]

Fixes #6288

HttpProvider set Content-Length to the JavaScript string length, which differs from the UTF-8 byte length for non-ASCII JSON. This can leave valid requests incomplete with Node's built-in fetch.

Remove the provider-generated header so fetch calculates the length of its string body. Explicit user headers, JSON encoding, and HTTP/RPC error handling remain unchanged.

Add 10 real localhost HTTP regression cases covering ASCII, Hangul, accents, surrogate pairs, combining characters, empty text, custom headers, an explicit correct length, HTTP errors, and RPC errors.

Validation:

• node .yarn/releases/yarn-4.6.0.cjs test:one rpc-provider: runner reports 134 passed, 0 failed, 1 existing skipped suite, with both native fetch and NODE_OPTIONS=--no-experimental-fetch.
• All 10 new cases pass. Against the original source, 7 fail and 3 pass, confirming regression sensitivity.
• Full yarn build (including TypeScript project checks) and changed-file ESLint pass.
• Built CJS/ESM HTTP consumers pass with both fetch implementations. Chrome 156 preserves exact Unicode echoes and byte lengths for both revisions.

The node-fetch fallback and browser paths already corrected/managed the header before this change. No live chain operations or full monorepo test run was used.