fix(mcp): do not let empty cdpHeaders from env override config file

[harrytran001]

Summary

• Preserve browser.cdpHeaders from config files when PLAYWRIGHT_MCP_CDP_HEADERS is unset.
• Avoid producing/merging empty cdpHeaders overrides from env parsing.
• Add regression test in tests/mcp/config.spec.ts.

[harrytran001]

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[dgozman]

It seems like a better solution could be replacing return previous || {}; with return previous; in the headerParser() function. Basically, returning undefined for the empty headers string. Then, usual logic with filtering undefined would just work. WDYT?

[harrytran001]

I agree. I made the change.

[github-actions]

Test results for "MCP"

5562 passed, 340 skipped

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Merge workflow run.

[dgozman]

Looks great, thank you!