fix(transport): fix unclosed quote in Endpoint.toString()

[Layau-code]

Describe what this PR does / why we need it

This PR fixes a malformed toString() output in the Endpoint class, where the host field was printed with an unclosed single quote (e.g., host='localhost, port=8900}). The unmatched quote causes confusing log messages. This change removes the stray quote for a clean format like host=localhost.

Does this pull request fix one issue?

Fixes #3573

Describe how you did it

Removed the opening single quote before the host value in the toString() method of com.alibaba.csp.sentinel.transport.heartbeat.Endpoint, so the field is now printed without quotes.

Describe how to verify it

1. Build with mvn clean package.

2. Run a demo that connects to Sentinel Dashboard (e.g., sentinel-demo-basic).

3. Check logs during startup or heartbeat — the output should be:

   Endpoint{protocol=HTTP, host=localhost, port=8900}

No unmatched quotes appear.

Special notes for reviews

• Purely cosmetic; no functional impact.
• Safe to omit quotes since host is typically a hostname or IP without special characters.
• Consistent with the style of other fields (protocol, port) which are also unquoted.

[CLAassistant]

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