chore: bump version to 1.2.18

[ruturaj-browserstack]

Summary

• Bumps @browserstack/mcp-server from 1.2.17 → 1.2.18 in package.json, package-lock.json, and server.json (nested packages[].version).
• Patch release covering changes merged since 1.2.17 (sub test plan tools, selfheal session/prompt fixes, accessibility password redaction, etc.).

Test plan

• CI passes (npm run build — lint + format + test + tsc).
• After merge, maintainer triggers the Release New Version workflow to publish v1.2.18 to npm.

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