feat: improve type assertion messages with actual ValueType

[baylesj]

When interacting with a Json::Value, numerous assertions correctly check that the value is of an expected type (e.g., an object or array) before accessing it. If there is a mismatch, the error is somewhat confusing as it only says "requires objectValue".

This commit improves the assertion messages by introducing a valueTypeToString helper, allowing the assertions to report the actual ValueType encountered (e.g., "requires objectValue, but found stringValue"). This will significantly streamline debugging for users experiencing type mismatch errors.

Fixes #1627

[coveralls]

Coverage Report for CI Build 25890872676

Coverage decreased (-0.2%) to 89.944%

Details

• Coverage decreased (-0.2%) from the base build.
• Patch coverage: 7 uncovered changes across 1 file (13 of 20 lines covered, 65.0%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
src/lib_json/json_value.cpp	20	13	65.0%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	2726
Covered Lines:	2602
Line Coverage:	95.45%
Relevant Branches:	2614
Covered Branches:	2201
Branch Coverage:	84.2%
Branches in Coverage %:	Yes
Coverage Strength:	23434.91 hits per line

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