fix(maticjs): disable HTTP keep-alive on metadata fetches to fix Premature close

[MaximusHaximus]

Problem

@maticnetwork/maticjs loads network config + contract ABIs at runtime from static.polygon.technology via node-fetch. Since Node 19 made HTTP keep-alive the default, node-fetch reuses a socket the CDN has idle-closed; the next gzip response dies mid-decompression as FetchError: Premature close, which surfaces as the misleading network <net> - <ver> is not supported. It reproduces 100% from datacenter/CI egress and intermittently elsewhere, and the retry added in #481 can't recover because every reused socket is already stale.

Fix

Pass node-fetch a keepAlive: false agent for these fetches (Node build only — window.fetch ignores agent). They run once per client init and are cached, so a fresh connection per request only costs a TLS handshake at startup. webpack resolve.fallback maps http/https to empty stubs so the web/UMD/standalone bundles still build (the browser branch never executes them).

Evidence

Isolated in CI on the same runner that fails today (5 attempts each, .../mainnet/v1/index.json): node-fetch default keep-alive 0/5; keepAlive:false 5/5. curl + Node's built-in fetch (undici) reach origin 30/30, confirming it's the node-fetch keep-alive path, not a network/CDN block.

Validation

pnpm --filter @maticnetwork/maticjs typecheck + build (all three webpack targets) + test (36 passing) green locally.

[claude]

Code review

No issues found. Checked for bugs and CLAUDE.md compliance.