Fix multi-instance support on same page

[matipojo]

Summary

• openIframe() now returns { iframe, iframeOrigin } so callers can post messages directly to a specific iframe instance.
• loadSidebar() sends sdk-widget-config directly to the instance's iframe via postMessage instead of using the shared postMessageToAngieIframe helper, preventing config cross-talk when multiple SDK instances coexist on the same page.
• Removed the postMessageToAngieIframe import from angie-mcp-sdk.ts since it's no longer needed there.

Test plan

• Updated existing widget-config tests to assert postMessage is called on the correct iframe with the correct origin
• Added test for mobile case where openIframe returns undefined — widget config is silently skipped
• Verified no regressions in other test suites

✨ PR Description

Purpose: Fix multi-instance support to enable multiple Angie SDK instances on the same page with proper iframe isolation and message routing.

Main changes:

• Added instance-based message filtering using instanceId in sidebar toggle events to prevent cross-instance interference
• Refactored openIframe to return iframe reference and origin, enabling direct postMessage instead of global helper
• Added path validation security check and configurable path option with ModeSwitcher and closeButton widget config

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[wiz-9a149474ff]

Wiz Scan Summary

Scanner	Findings
Vulnerabilities	-
Sensitive Data	-
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IaC Misconfigurations	-
SAST Findings	1
Software Management Findings	-
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