libfetch: retain run dependency helpers for IndexedDB

[kypp]

Fix: keep run dependency helpers when FETCH uses IndexedDB

Fetch.init() calls addRunDependency/removeRunDependency when FETCH_SUPPORT_INDEXEDDB is enabled. In -s MODULARIZE=1 builds those helpers can be DCE'd, causing ReferenceError: addRunDependency is not defined at runtime.

This change declares $addRunDependency and $removeRunDependency as deps of $Fetch (guarded by FETCH_SUPPORT_INDEXEDDB), so the helpers are retained without requiring extra user flags.

Fixes #26008

Repro:

emcc main.c -s FETCH=1 -s MODULARIZE=1 -o out.mjs

Open the output in a browser:

• Before: ReferenceError: addRunDependency is not defined
• After: no error

[Copilot]

Pull request overview

This PR fixes a dead code elimination (DCE) issue in MODULARIZE builds where the Fetch API uses IndexedDB. The Fetch.init() function calls addRunDependency/removeRunDependency when IndexedDB support is enabled, but these helper functions were being removed by DCE, causing a ReferenceError at runtime.

• Adds $addRunDependency and $removeRunDependency as dependencies of $Fetch when FETCH_SUPPORT_INDEXEDDB is enabled
• Prevents DCE from removing these runtime helper functions in MODULARIZE builds

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