fix: allow empty CLI env values

[pragnyanramtha]

Summary

• allow -e KEY= in the Inspector launcher instead of rejecting empty environment variable values
• keep rejecting malformed values without an = and empty keys
• add CLI regression coverage for passing an empty env var through to a stdio MCP server

Validation

• npm install
• npm run build-cli
• npx prettier --check cli/src/cli.ts cli/__tests__/cli.test.ts
• export PATH=/home/ubuntu/gh-pr/inspector-main/node_modules/.bin:$PATH; cd cli && npm run test:cli -- -t "empty environment variable values"
• export PATH=/home/ubuntu/gh-pr/inspector-main/node_modules/.bin:$PATH; timeout 30s node cli/build/cli.js tsx /home/ubuntu/gh-pr/inspector-main/cli/__tests__/helpers/test-server-stdio.ts -e EMPTY_ENV= --cli --method resources/read --uri test://env
• git diff --check

Note: running the full CLI suite without adding the repo-local node_modules/.bin to PATH timed out in this local checkout because the test MCP server command could not resolve tsx; the targeted regression passes when tsx is resolvable.

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