Treat dynamic imports with rejection handlers as optional dependencies

[robhogan]

Summary:
Fixes: #1681

Extends the optional-dependency heuristic in collectDependencies to recognise dynamic imports that attach a rejection handler — either .catch(handler) or .then(_, handler) — anywhere in an unbroken promise chain rooted at the import() call.

Today, transformer.allowOptionalDependencies only treats require() / await import() as optional when wrapped in a try block. Library code that relies on a chained catch, e.g.

import('node:diagnostics_channel')
  .then(dc => { ... })
  .catch(() => { ... });

(seen in lru-cache for instance: https://unpkg.com/lru-cache@11.3.5/dist/esm/diagnostics-channel.js ) still fails the build with an unresolvable-module error, even though the developer has clearly opted into a runtime fallback.

This makes a pragmatic extension to collectDependencies to walk up the chain from the import() call and treat the dependency as optional if any chained call provides a rejection handler. The walk stops as soon as the chain is broken, keeping the heuristic local to the import.

Changelog:

 - **[Fix]**: Treat `import().catch()`, etc. as optional under `resolver.allowOptionalDependencies`

Reviewed By: huntie

Differential Revision: D102145525

[meta-codesync]

@robhogan has exported this pull request. If you are a Meta employee, you can view the originating Diff in D102145525.

[meta-codesync]

This pull request has been merged in 23e4214.