feat(InstallWizard): support fieldMapping prop (dynamic field + value mappings)

[dionlow]

Context

The classic Configure variant fully supports the developer-supplied fieldMapping prop (<InstallIntegration fieldMapping={...}>); the wizard (variant="wizard") ignored it entirely — grep for fieldMapping/valueMapping/mappedValues across InstallWizard/ and headless/ returned zero matches. The wizard sourced mapping rows only from the amp.yaml manifest, so anything declared only in the prop never rendered.

This brings the wizard to parity: a wizard install consuming a fieldMapping prop now produces the same selectedFieldMappings + selectedValueMappings config as the classic flow. Part of wizard parity (ENG-4056).

The data model

FieldMapping = { [objectName]: DynamicMappingsInputEntry[] }. Three entry shapes, three behaviors:

Entry shape	mapToName	fieldName	mappedValues	Consumer action
field + values	✅	❌	✅	Pick a field, then map its values
value-only	❌	✅ (fixed)	✅	Only map values (field pre-known)
field-only	✅	❌	❌	Only pick a field

Changes

• headless (useConfigHelper.tsx): add getValueMapping/setValueMapping to ReadObjectHandlers. Dynamic field mappings reuse the existing setFieldMapping.
• derivation (useObjectMappings.ts): merge manifest map fields with the prop — prop dynamic field mappings (mapToName, no fieldName) fold into optional rows with prop precedence on collisions; prop entries with mappedValues become value-mapping units.
• UI (value-mappings/): per-entry value-mapping blocks rendered under field mappings on the same Mappings page. Field-mapped units resolve their field from the user's selection; fixed-field units map values directly. Options/validation mirror classic. Fully derived — no useEffect, no seeding.
• navigation (subPageUtils.ts, useSubPageNavigation.ts, ObjectTabs.tsx): the existing Mappings sub-page now appears when mappings come only from the prop. No new sub-page added.
• submit (buildSubmissionConfig.ts → ReviewStep.tsx): derived projection prunes stale selectedValueMappings (fields no longer resolved) at create time instead of eager clearing, preserving user picks on field toggle. Review summary now includes prop-dynamic field mappings.

Verification

• ✅ tsc --noEmit clean
• ✅ yarn lint — 0 errors
• ✅ yarn build — full ESM/CJS/dts build succeeds

Not included

• Automated tests: the repo has no working jest TS setup (no babel preset / alias mapping / testing-library; yarn test reports "No tests found"). Standing up that infra + the buildSubmissionConfig/useConfigHelper unit tests is a separate change. Manual/Storybook verification against a reference fieldMapping prop still pending.
• Manifest-driven value mappings (feature is prop-driven, matching classic); wizard update/edit-existing flow (wizard is create-only today).

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