test: add Gherkin test definition for getCall operation (fixes #78)

[YadingFang]

What type of PR is this?

• tests
  What this PR does / why we need it:
  Fixes Missing Gherkin test definition for getCall operation #78

Adds a Gherkin test definition for the getCall operation (GET /calls/{callId}), which was missing from code/Test_definitions/.

Scenarios covered:

• 200 — successful retrieval of call details (validates Call schema and x-correlator header)
• 400 — invalid request / malformed path parameter
• 401 — authentication failure
• 403 — authorization failure (insufficient permissions)
• 404 — unknown call identifier

[github-actions]

🦙 MegaLinter status: ✅ SUCCESS

Descriptor	Linter	Files	Fixed	Errors	Elapsed time
✅ ACTION	actionlint	2		0	0.01s
✅ API	spectral	1		0	1.63s
✅ GHERKIN	gherkin-lint	4		0	1.14s
✅ REPOSITORY	git_diff	yes		no	0.01s
✅ REPOSITORY	secretlint	yes		no	0.66s
✅ YAML	yamllint	1		0	0.44s

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[XunliYang]

LGTM