fix(samples/kotlin): send customer email for USDB embedded wallets

[pengying]

The platform requires a customer email when it supports USDB embedded
wallets. Wire email through the customer create builder and add it to
the sample's default request body. The Grid API validates domain
deliverability, so the default uses a real domain (gmail.com).

Co-Authored-By: Claude Opus 4.8 (1M context) noreply@anthropic.com

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2 Skipped Deployments

Project	Deployment	Actions	Updated (UTC)
grid-flow-builder	Ignored	Preview	Jun 18, 2026 6:13pm
grid-wallet-demo	Ignored	Preview	Jun 18, 2026 6:13pm

[pengying]

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This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
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• fix(samples/frontend): floor default quote amount to corridor minimum #605
• fix(samples/frontend): scale default quote amount by source currency decimals #604
• fix(samples/kotlin): select crypto network when funding a quote from USDC #603
• feat(samples/kotlin): add exchange-rates lookup flow #602
• feat(samples/kotlin): add USD → USDC wallet payout flow #593
• fix(samples/kotlin): send customer email for USDB embedded wallets #590 👈 (View in Graphite)
• feat(samples/kotlin): support GRID_API_BASE_URL override #589
• main

This stack of pull requests is managed by Graphite. Learn more about stacking.

[greptile-apps]

Greptile Summary

This PR adds email support to the customer-creation flow for platforms that require it when USDB embedded wallets are enabled. The Grid API validates the domain's MX records, so the default sample value uses a real domain with a random +suffix to keep each run distinct.

• Frontend (CreateCustomer.tsx): Appends a jack.doe+<random>@gmail.com email to the default JSON body, with a comment explaining the MX-validation requirement.
• Kotlin sample (Customers.kt): Reads email from the incoming JSON and forwards it to the CreateCustomerRequest builder using the same optional-chaining pattern already used for all other fields.

Confidence Score: 5/5

Safe to merge — small, additive changes that follow existing patterns in both files.

Both changes are minimal and correct: the frontend adds a well-formed sample email with a clear explanatory comment, and the Kotlin server forwards the field through the builder using the same optional-chaining idiom already used for every other field. No existing behaviour is modified.

No files require special attention.

Important Files Changed

Filename	Overview
samples/frontend/src/steps/CreateCustomer.tsx	Adds a randomly-suffixed gmail.com email to the default request body, with a comment explaining the MX-validation requirement; pattern is consistent with the existing platformCustomerId random suffix.
samples/kotlin/src/main/kotlin/com/grid/sample/routes/Customers.kt	Wires the optional email field from the incoming JSON body to the customer builder, matching the style of every other optional field in the builder block.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant Browser as Frontend (React)
    participant Kotlin as Kotlin Sample Server
    participant Grid as Grid API

    Browser->>Kotlin: "POST /api/customers { platformCustomerId, email, fullName, ... }"
    Kotlin->>Kotlin: "optText("email")?.let { email(it) }"
    Kotlin->>Grid: customers.create(CustomerCreateParams)
    Grid->>Grid: Validate email domain MX records
    Grid-->>Kotlin: Customer response (JSON)
    Kotlin-->>Browser: 201 Created (JSON)

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%%{init: {'theme': 'base', 'themeVariables': {"darkMode": true, "background": "#0d1117", "primaryColor": "#21262d", "primaryTextColor": "#e6edf3", "primaryBorderColor": "#8b949e", "lineColor": "#8b949e", "textColor": "#e6edf3", "edgeLabelBackground": "#161b22", "actorBkg": "#21262d", "actorBorder": "#8b949e", "actorTextColor": "#e6edf3", "actorLineColor": "#8b949e", "signalColor": "#8b949e", "signalTextColor": "#e6edf3", "noteBkgColor": "#373320", "noteBorderColor": "#d4a72c", "noteTextColor": "#f0e6c0", "labelBoxBkgColor": "#21262d", "labelBoxBorderColor": "#8b949e", "labelTextColor": "#e6edf3", "loopTextColor": "#e6edf3", "activationBkgColor": "#30363d", "activationBorderColor": "#8b949e"}}}%%
sequenceDiagram
    participant Browser as Frontend (React)
    participant Kotlin as Kotlin Sample Server
    participant Grid as Grid API

    Browser->>Kotlin: "POST /api/customers { platformCustomerId, email, fullName, ... }"
    Kotlin->>Kotlin: "optText("email")?.let { email(it) }"
    Kotlin->>Grid: customers.create(CustomerCreateParams)
    Grid->>Grid: Validate email domain MX records
    Grid-->>Kotlin: Customer response (JSON)
    Kotlin-->>Browser: 201 Created (JSON)

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_{Reviews (2): Last reviewed commit: "fix(samples/kotlin): send customer email..." | Re-trigger Greptile}

[restamp-bot]

4019a29 is a pure rebase onto f206e35. Approving based on @jklein24's previous approval of a565834.

[restamp-bot]

4019a29 is a pure rebase onto f206e35. Approving based on @jklein24's previous approval of a565834.