💥 Merge asyncToStringMethod into toStringMethod

[dubzzz]

Description

AI-agent disclosure: this PR was authored by an automated agent (Claude Code) and has not been line-by-line reviewed by a human before submission.

In the past we used to have two symbols to declare a custom serializer on an instance: toStringMethod for synchronous contexts and asyncToStringMethod for asynchronous ones.

The two got merged into a single one: toStringMethod. Its serializer can now return either a string or a promise of string. In other words, users previously relying on asyncToStringMethod just have to declare the same serializer on toStringMethod.

Breaking changes (impact flagged as major via a changeset):

• asyncToStringMethod, WithAsyncToStringMethod and hasAsyncToStringMethod are dropped from the public API. Migration: replace asyncToStringMethod by toStringMethod.
• WithToStringMethod now declares () => string | Promise<string> instead of () => string.
• toStringMethod is now invoked even in synchronous contexts when its output turns out to be a promise (its output simply cannot be leveraged there and fast-check falls back to its default serialization). Previously asyncToStringMethod was never called in synchronous contexts.

Promise-based outputs keep the exact same constraints as asyncToStringMethod had: they are only exploited by asynchronous properties and must resolve almost instantly.

This PR follows the same direction as previous merges of the sync/async pairs (see the merge of asyncReporter into reporter in #7178): with asynchronous properties being the sole way to create properties, keeping two symbols for the same concern was no longer justified.

The PR is focused on that single concern. Internal users of the symbols were updated accordingly: func, streamOf and commands now expose a single toStringMethod returning a string when serialization can be done synchronously and a promise otherwise. Existing tests on stringify/asyncStringify and CommandWrapper were adapted to the merged symbol — among others they check that Promise-based outputs are used in asynchronous contexts and properly fall back in synchronous ones, and they would have failed without this PR. Documentation (custom-reports.md) was updated too.

Checklist

— Don't delete this checklist and make sure you do the following before opening the PR

• I have a full understanding of every line in this PR — whether the code was hand-written, AI-generated, copied from external sources or produced by any other tool
• I flagged the impact of my change (minor / patch / major) either by running pnpm run bump or by following the instructions from the changeset bot
• I kept this PR focused on a single concern and did not bundle unrelated changes
• I followed the gitmoji specification for the name of the PR, including the package scope (e.g. 🐛(vitest) Something...) when the change targets a package other than fast-check
• I added relevant tests and they would have failed without my PR (when applicable)

🤖 Generated with Claude Code

https://claude.ai/code/session_019H4rz5zF7PTMQK6JCFWh9o

---

Generated by Claude Code

[changeset-bot]

🦋 Changeset detected

Latest commit: 734f883

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 5 packages

Name	Type
fast-check	Major
@fast-check/ava	Patch
@fast-check/jest	Patch
@fast-check/vitest	Patch
@fast-check/worker	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[pkg-pr-new]

• @fast-check/examples

@fast-check/ava

npm i https://pkg.pr.new/@fast-check/ava@7181

fast-check

npm i https://pkg.pr.new/fast-check@7181

@fast-check/jest

npm i https://pkg.pr.new/@fast-check/jest@7181

@fast-check/packaged

npm i https://pkg.pr.new/@fast-check/packaged@7181

@fast-check/poisoning

npm i https://pkg.pr.new/@fast-check/poisoning@7181

@fast-check/vitest

npm i https://pkg.pr.new/@fast-check/vitest@7181

@fast-check/worker

npm i https://pkg.pr.new/@fast-check/worker@7181

commit: 734f883

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.50%. Comparing base (a22e3fb) to head (734f883).

Additional details and impacted files

@@               Coverage Diff               @@
##           next-v4_9_0    #7181      +/-   ##
===============================================
- Coverage        95.50%   95.50%   -0.01%     
===============================================
  Files              206      206              
  Lines             5274     5272       -2     
  Branches          1470     1471       +1     
===============================================
- Hits              5037     5035       -2     
  Misses             229      229              
  Partials             8        8              

Flag	Coverage Δ
tests	95.50% <100.00%> (-0.01%)	⬇️

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