write/point: run convertField when AddField re-sets an existing key

[c-tonneslan]

Found while reading api/write/point.go.

AddField runs the input value through convertField when appending a new field, so e.g. int32(5) lands in the point as int64(5), a time.Time as its RFC3339Nano string, etc. But the re-set path on the existing key did:

m.fields[i].Value = v

with the raw v. So the canonical-type guarantee quietly disappeared on update. Easiest to see with non-int64 ints, where the line-protocol encoder downstream expects int64:

p := NewPointWithMeasurement("m")
p.AddField("x", int32(1))   // stored as int64(1) via convertField
p.AddField("x", int32(2))   // stored as int32(2)  !!

Pulled the convertField call out front so both branches use the same value. Added a regression test on the int32 case.

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.99%. Comparing base (67f504e) to head (30aacbc).
⚠️ Report is 10 commits behind head on master.
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #428   +/-   ##
=======================================
  Coverage   92.99%   92.99%           
=======================================
  Files          25       25           
  Lines        2326     2327    +1     
=======================================
+ Hits         2163     2164    +1     
  Misses        123      123           
  Partials       40       40           

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