docs(api): align /calls with OpenAPI

[YadingFang]

What type of PR is this?

Add one of the following kinds:

• documentation

What this PR does / why we need it:

This PR improves and aligns the /calls documentation with the current ClickToDial API behavior and CAMARA expectations by:

Adding a CloudEvents payload example to illustrate call status change notifications.

Documenting 422 business error responses and their codes for common validation/eligibility cases.

Adding a call-flow diagram to clarify the expected call session lifecycle and notification flow.

These updates make the specification easier to understand and validate, and provide clearer guidance for implementers and testers.

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ ACTION	actionlint	2		0	0.01s
✅ API	spectral	1		0	1.9s
✅ GHERKIN	gherkin-lint	3		0	1.24s
✅ REPOSITORY	git_diff	yes		no	0.02s
✅ REPOSITORY	secretlint	yes		no	0.75s
✅ YAML	yamllint	1		0	0.6s

See detailed report in MegaLinter reports

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[XunliYang]

LGTM

[Wojiaozhenghao]

approve