Improve json conversion and stringify behavior

[bobzhang]

Motivation

• Consolidate deprecated Json helpers into a dedicated module for API clarity and consistency.
• Prevent surprising conversions by rejecting fractional numbers when decoding Int/UInt.
• Fix stringify/escape behavior to avoid unwanted indentation/newlines and improve common-case performance for unescaped strings.
• Add missing documentation for JsonPath helper APIs to improve discoverability.

Description

• Moved legacy accessors (Json::as_*, Json::item, Json::value) out of the main module into json/deprecated.mbt and left deprecation hints in place.
• Tightened from_json implementations for Int and UInt to reject fractional numbers by checking n == n.trunc() and added corresponding tests in json/from_json_test.mbt that assert fractional values fail with an expected integer error.
• Adjusted Json::stringify to not emit a leading indent for objects until the first property is written and to avoid emitting a trailing indent for empty objects when a replacer filtered out members, fixing compact output when Replacer::keep([]) is used.
• Added a fast-path to escape that returns the original string when no characters require escaping to avoid extra allocation/work in the common case.
• Added brief doc comments for JsonPath and its add_index/add_key helpers and added a unit test verifying replacer + indentation behavior in json/json_test.mbt.

Testing

• Ran moon info to update package metadata and ensure interfaces are consistent, which completed successfully.
• Ran moon fmt to format changes, which completed successfully.
• Ran the full test suite with moon test, and all tests passed: Total tests: 5632, passed: 5632, failed: 0.

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Codex Task

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[coveralls]

Pull Request Test Coverage Report for Build 2569

Details

• 14 of 14 (100.0%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.005%) to 95.932%

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Files with Coverage Reduction	New Missed Lines	%
json/json.mbt	1	99.12%

Totals
Change from base Build 2548:	-0.005%
Covered Lines:	11909
Relevant Lines:	12414

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💛 - Coveralls

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 6 additional findings.