[fix] dataconv: map json.Number to Int/Float and decode JSON exactly (LET-28)

[vt128]

Closes the JSON-number gaps surfaced by the diagnosis in #128 — thanks @phughes-scwx for the clear write-up and repro.

What

Two related gaps in how dataconv turns JSON numbers into Starlark values:

1. Marshal errored on json.Number. It had no json.Number case, so the value fell through to unrecognized type. A caller decoding raw JSON into map[string]interface{} with dec.UseNumber() — the way to keep the int/float distinction that a plain json.Unmarshal collapses to float64 — could not then Marshal it.

2. UnmarshalStarlarkJSON lost precision. It decoded without UseNumber() and re-derived int-ness from float64, silently saturating integers past 2^53. It is on the lib/http request-body path, so real JSON ids were affected.

Fix

• Marshal maps json.Number by literal form: an integer literal → Int (with *big.Int beyond int64), otherwise → Float. This matches json.decode, serial, and dataconv's own GoToStarlarkViaJSON. Mapping to a string (as proposed in Add support for json.Number to dataconv.Marshal #128) would make Marshal the only converter that turns a number into a non-number and would break value == 6/arithmetic in scripts. Marshal also now accepts *big.Int (the inverse of Unmarshal, so a round-trip stays exact).
• UnmarshalStarlarkJSON decodes with UseNumber() and the same number-preserving logic, and re-imposes the single-document strictness the previous json.Unmarshal had.

Note on @phughes-scwx / #128

We can't merge #128 directly (the string mapping), but the gap it identified is real and we've landed it here with the Int/Float mapping and credited the report. @phughes-scwx — does this implementation work for your use case? Happy to adjust.

Behavior change

With the unified literal-form rule, a whole float literal like 6.0 now decodes to Float (previously UnmarshalStarlarkJSON's heuristic returned Int). Common cases (6→Int, 6.5→Float, large ints→exact) are unchanged; no test pinned the old 6.0→int behavior. interface.go doc updated accordingly.

Test-first

TestMarshal gains json.Number/*big.Int cases; TestUnmarshalStarlarkJSONNumberFidelity pins exact big-int decoding (including the dict/HTTP-body path). Fail before the fix.

Verification

go test -race -count=2 ./... (incl. lib/http), go vet, gofmt -l clean, Docker golang:1.19 race run green.

Requirement: LET-28 · Refs #128

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 22 complexity · 1 duplication

Metric	Results
Complexity	22
Duplication	1

View in Codacy

🟢 Coverage 98.15% diff coverage · +0.04% coverage variation

Metric	Results
Coverage variation	✅ +0.04% coverage variation (-1.00%)
Diff coverage	✅ 98.15% diff coverage

View coverage diff in Codacy

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (433eae0)	7692	7276	94.59%
Head commit (fc3a6c9)	7744 (+52)	7328 (+52)	94.63% (+0.04%)

Coverage variation is the difference between the coverage for the head and common ancestor commits of the pull request branch: <coverage of head commit> - <coverage of common ancestor commit>

Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#167)	54	53	98.15%

Diff coverage is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: <covered lines added or modified>/<coverable lines added or modified> * 100%

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[codecov]

Codecov Report

❌ Patch coverage is 96.77419% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 93.42%. Comparing base (433eae0) to head (fc3a6c9).
⚠️ Report is 3 commits behind head on master.

Files with missing lines	Patch %	Lines
dataconv/helper.go	92.85%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #167      +/-   ##
==========================================
+ Coverage   93.39%   93.42%   +0.03%     
==========================================
  Files          49       49              
  Lines        6177     6206      +29     
==========================================
+ Hits         5769     5798      +29     
+ Misses        260      259       -1     
- Partials      148      149       +1     

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