(fix) O3-5607: Prevent NPE when assignTicket payload fields are missing

[sanks011]

Requirements

• This PR has a title that briefly describes the work done including the ticket number. If there is a ticket, make sure your PR title includes a conventional commit label. See existing PR titles for inspiration.
• [] My work is based on designs, which are linked or shown either in the Jira ticket or the description below. (N/A - Critical backend logic fix)
• My work includes tests or is validated by existing tests.

Summary

This PR fixes a critical NullPointerException (HTTP 500 API crash) and completes hardening of the Legacy1xRestController/assignTicketToServicePoint endpoint. Three issues were addressed based on Copilot AI review:

1. NullPointerException on missing JSON keys (original bug)
Jackson's get(property) returns null for missing fields — calling .textValue() or .isEmpty() on null crashed the endpoint.

2. Silent type coercion (Copilot comment 1)
The initial fix used asText("") which would coerce non-string types (e.g. {"status": 123}) into valid strings, violating the API contract. Now using explicit isTextual() + isMissingNode() + isNull() checks per Copilot's suggestion — non-string values are now rejected with 400.

3. Malformed JSON body crash (Copilot comment 2)
mapper.readTree("") throws JsonProcessingException for blank or garbage input, which bubbled up as a 500. Now caught and returned as 400 Bad Request.

4. Unit tests added (Copilot comment 3)
Added Legacy1xRestControllerAssignTicketTest covering 8 scenarios: empty body, invalid JSON, missing fields, explicit null values, non-string types, empty strings, and the valid happy path.

Related Issue

https://issues.openmrs.org/browse/O3-5607

Other

No UI changes. Backend only.

[Copilot]

Pull request overview

Fixes a NullPointerException in the legacy /queueutil/assignticket endpoint when expected JSON fields are missing or null, so the endpoint returns a 400 instead of crashing with a 500.

Changes:

• Replaced actualObj.get(...).textValue() with actualObj.path(...).asText("") for safer JSON field extraction.
• Ensures missing/null payload fields fall through to existing required-field validation (isEmpty() -> 400).

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