feat(analytics): add service account authentication support

[lajohn4747]

Summary

Add service account authentication for /import endpoint and feature flags.

• Add ServiceAccountCredential class with field validation
• Update MixpanelAPI to support service account auth for /import
• Add service account support to feature flags (BaseFlagsConfig, providers)
• Automatically inject credentials from MixpanelAPI Builder into flags configs
• Add basic tests for auth methods and backward compatibility
• Update README with usage examples

[linear-code]

MULTI-564

[greptile-apps]

Confidence Score: 5/5

The change is additive and opt-in; all existing call sites pass null for credentials and follow the token-based path unchanged.

Backward compatibility is maintained through overloaded constructors, and input validation in ServiceAccountCredential catches the most dangerous misuse at construction time.

No files require special attention.

Important Files Changed

Filename	Overview
src/main/java/com/mixpanel/mixpanelapi/ServiceAccountCredential.java	New credential holder with field validation including colon-in-username guard for RFC 7617 compliance.
src/main/java/com/mixpanel/mixpanelapi/MixpanelAPI.java	Threads credentials through Builder and all constructors; sendImportData correctly switches between token and service-account Basic Auth and appends project_id.
src/main/java/com/mixpanel/mixpanelapi/featureflags/provider/BaseFlagsProvider.java	Stores credentials; getWithAuth correctly switches auth header between service-account and token modes.
src/test/java/com/mixpanel/mixpanelapi/MixpanelAPITest.java	Adds credential validation tests and Builder tests; testServiceAccountNotUsedForTracking and testBuilderWithCredentials are redundant duplicates.
src/main/java/com/mixpanel/mixpanelapi/featureflags/provider/LocalFlagsProvider.java	Adds project_id to definitions URL when credentials present; delegates to new 4-arg super constructor.
src/main/java/com/mixpanel/mixpanelapi/featureflags/provider/RemoteFlagsProvider.java	Adds project_id to flags URL when credentials present; mirrors LocalFlagsProvider pattern.
README.md	Adds service account usage examples; README notes now correctly describe token still present on flag URLs.

Sequence Diagram

%%{init: {'theme': 'neutral'}}%%
sequenceDiagram
    participant App
    participant Builder as MixpanelAPI.Builder
    participant API as MixpanelAPI
    participant Flags as Flags Providers
    participant MP as Mixpanel Server

    App->>Builder: credentials(ServiceAccountCredential)
    Builder->>API: build()
    API->>Flags: new Provider(..., credentials)
    App->>API: deliver(import messages)
    API->>MP: "POST /import?project_id=X"
    App->>Flags: getFlags()
    Flags->>MP: "GET /flags?token=T&project_id=X"
    MP-->>Flags: flag definitions
    Flags-->>App: flag value

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sequenceDiagram
    participant App
    participant Builder as MixpanelAPI.Builder
    participant API as MixpanelAPI
    participant Flags as Flags Providers
    participant MP as Mixpanel Server

    App->>Builder: credentials(ServiceAccountCredential)
    Builder->>API: build()
    API->>Flags: new Provider(..., credentials)
    App->>API: deliver(import messages)
    API->>MP: "POST /import?project_id=X"
    App->>Flags: getFlags()
    Flags->>MP: "GET /flags?token=T&project_id=X"
    MP-->>Flags: flag definitions
    Flags-->>App: flag value

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