Align ClouEvent schema, security schemes and clean up intro/error codes

[YadingFang]

…

What type of PR is this?

Add one of the following kinds:

• correction
• cleanup

What this PR does / why we need it:

• CloudEvent: Removed subject property (redundant with data), updated required fields and examples.
• Security: Added notificationsBearerAuth security scheme for subscription APIs.
• Cleanup: Removed Introduction section containing application-level descriptions (e.g., number masking, pool of minutes).
• Error Codes: Removed INSUFFICIENT_BALANCE and RESTRICTED_DESTINATION as they are not currently supported by the API.

Special notes for reviewers:

Address feedback from release review (PR #65) and align with Commonalities 0.7.0 direction.

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ ACTION	actionlint	2		0	0.01s
✅ API	spectral	1		0	1.71s
✅ GHERKIN	gherkin-lint	3		0	1.04s
✅ REPOSITORY	git_diff	yes		no	0.02s
✅ REPOSITORY	secretlint	yes		no	0.76s
✅ YAML	yamllint	1		0	0.47s

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