Migrate ECB tutorial from gulp to heft build system

[Ashlesha-MSFT]

Category

• Content fix

Related issues

• fixes SPFx Docs Update for Heft toolchain: migrate-from-ecb-to-spfx-extensions.md #10587

What's in this Pull Request?

Migrate ECB tutorial from gulp to heft build system

• Updated build commands from gulp to heft (start, build, package-solution)
• Improved TypeScript code with null safety and modern best practices
• Removed unused imports (Dialog, strings)
• Added proper case block scoping with braces
• Updated serve.json configuration for debugging
• Corrected elements.xml with proper GUID references
• Updated documentation with working code examples

[learn-build-service-prod]

PoliCheck Scan Report

The following report lists PoliCheck issues in PR files. Before you merge the PR, you must fix all severity-1 and severity-2 issues. The AI Review Details column lists suggestions for either removing or replacing the terms. If you find a false positive result, mention it in a PR comment and include this text: #policheck-false-positive. This feedback helps reduce false positives in future scans.

✅ No issues found

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[learn-build-service-prod]

Learn Build status updates of commit 87b338e:

✅ Validation status: passed

File	Status	Preview URL	Details
docs/images/ecb-menu-classic-output.png	✅Succeeded	View
docs/images/spfx-ecb-extension-add-app.png	✅Succeeded	View
docs/images/spfx-ecb-extension-output.png	✅Succeeded	View
docs/spfx/extensions/guidance/migrate-from-ecb-to-spfx-extensions.md	✅Succeeded	View

For more details, please refer to the build report.