Optimize substring copies in the Message.extractField hot path

[leonchen83]

Motivation

While profiling temporary object allocations with async-profiler, we found that
Message.extractField is one of the hottest allocation paths. Parsing a single
message copies the raw message data several times via String.substring,
String.getBytes and Arrays.copyOf:

• one substring of the tag per field, for IntConverter.convert
• one substring of the value per field, for every StringField
• for data fields (RawData etc.), substring().getBytes() on every scan
  iteration to measure the encoded length, copying the whole span repeatedly
  (O(n²) for large payloads)

hedger1-alloc-20260930-151550.html

Changes

This PR optimizes the substring allocations inside the hot method
extractField.

Core idea: instead of copying the value eagerly, we keep the original String
plus the value's offset/length, and only perform the substring when the value
is actually needed.

• StringField — keeps the raw message String together with offset and
  length, and only does the substring when getValue() is called, releasing
  the reference to the raw message afterwards. equals/hashCode compare and
  hash the value range directly, without triggering a substring.

• All converters gained convert(String, offset, length) — type conversion
  reads the given range of the original String directly, so no substring is
  needed before converting. The typed getters of FieldMap
  (getInt/getDouble/getDecimal, ...) go through this path and are
  allocation-free.

• Message.extractField — builds StringField instances from the value
  range instead of substringing every field. The length measurement for data
  fields (RawData etc.) no longer uses substring().getBytes() either; the
  encoded byte length is computed directly over the original String.

Compatibility

Field values, error messages and parse positions are byte-for-byte identical to
the previous implementation, including the null-value contract covered by
MessageTest/FieldMapTest. Non-String fields and unknown charsets fall back to
the original code paths.

Result

async-profiler allocation profile after this change — the substring,
String.getBytes and Arrays.copyOf frames in extractField are gone:

hedger1-alloc-20260930-153207.html

[chrjohn]

@leonchen83 thanks for the PR. 👍
Could you please remove the unrelated commits? Thank you

[codecov]

Codecov Report

❌ Patch coverage is 52.25225% with 106 lines in your changes missing coverage. Please review.
✅ Project coverage is 70.19%. Comparing base (db43e4d) to head (75fcdd3).
⚠️ Report is 4 commits behind head on master.

Files with missing lines	Patch %	Lines
...kfixj-base/src/main/java/quickfix/StringField.java	54.09%	18 Missing and 10 partials ⚠️
...ava/quickfix/field/converter/DecimalConverter.java	43.90%	12 Missing and 11 partials ⚠️
quickfixj-base/src/main/java/quickfix/Message.java	24.00%	17 Missing and 2 partials ⚠️
...a/quickfix/field/converter/CharArrayConverter.java	50.00%	4 Missing and 5 partials ⚠️
...java/quickfix/field/converter/DoubleConverter.java	58.82%	3 Missing and 4 partials ⚠️
...n/java/quickfix/field/converter/CharConverter.java	33.33%	3 Missing and 3 partials ⚠️
...uickfixj-base/src/main/java/quickfix/FieldMap.java	70.58%	5 Missing ⚠️
...ava/quickfix/field/converter/BooleanConverter.java	50.00%	2 Missing and 3 partials ⚠️
...in/java/quickfix/field/converter/IntConverter.java	83.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1340      +/-   ##
============================================
- Coverage     70.80%   70.19%   -0.61%     
- Complexity     2271     2306      +35     
============================================
  Files           161      161              
  Lines          9077     9241     +164     
  Branches       1192     1239      +47     
============================================
+ Hits           6427     6487      +60     
- Misses         2197     2265      +68     
- Partials        453      489      +36     

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[leonchen83]

Already done — I force-pushed the branch and the PR now contains only the 3
relevant commits. Please refresh and take another look. Let me know if you'd
prefer them squashed into a single commit.