fix(client): preserve Request options in proxy fetch

[go165]

Summary

• Normalize proxy fetch inputs with new Request(input, init) before serializing them for /fetch
• Preserve Request method, headers, and body instead of forwarding only Request.url
• Add regression coverage for Request inputs and explicit init overrides

Closes #1207.

Validation

• git diff --check
• /home/ubuntu/modelcontextprotocol-inspector/clients/web/node_modules/.bin/prettier --check client/src/lib/proxyFetch.ts client/src/lib/__tests__/proxyFetch.test.ts
• Manual Node 24 check that new Request() preserves POST method, lower-cased headers, and text body for the reported Request-input case

I attempted the focused Jest command npm test -- --runTestsByPath src/lib/__tests__/proxyFetch.test.ts --runInBand, but this fresh worktree did not have jest installed. A scoped npm ci --workspace=client --include-workspace-root=false --ignore-scripts then failed on registry download with ECONNRESET for xml-name-validator-4.0.0.tgz, so I did not keep retrying registry downloads.

[go165]

Validation update for the latest head 3746d533:

• npm ci --workspace=client --include-workspace-root=false --ignore-scripts
• npm test -- --runTestsByPath src/lib/__tests__/proxyFetch.test.ts --runInBand (12 passed)
• npx prettier --check client/src/lib/proxyFetch.ts client/src/lib/__tests__/proxyFetch.test.ts
• git diff --check origin/main...HEAD

GitHub reports the pushed commit signature as verified.