refactor(plugin-rsc): convert hooks to nested handler form

[Copilot]

Preparation for #957 by converting all resolveId, load, and transform hooks in plugin-rsc to nested handler form.

Changes

Converted 18 hooks from direct function form to nested handler object:

// Before
resolveId(source) { ... }
load(id) { ... }
async transform(code, id) { ... }

// After
resolveId: {
  handler(source) { ... }
}
load: {
  handler(id) { ... }
}
transform: {
  async handler(code, id) { ... }
}

Affected hooks:

• 5 resolveId hooks (rpc-client, assets-manifest, client-references, encryption-key, css-virtual)
• 7 load hooks (rpc-client, assets-manifest, client-references, client-package, encryption-key, css-virtual, importer-resources)
• 6 transform hooks (load-environment-module, bootstrap-script, use-client, use-server, css-export, importer-resources)

Pure refactoring—no functional changes. Enables hook filtering and composition patterns required by #957.

Original prompt

I'd like to prepare for #957 by first pure refactoring of making all (or most) of (reoslveId/load/transform) to have nested "handler" form
{ name: 'foo', transform() {} } => { name: 'foo', transform: { handler() {} } }

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