feat: add internal flag to Endpoint create params (VT-9534)

[revanth-plivo]

Summary

Adds an optional isInternal (bool?) parameter to EndpointInterface.Create and CreateAsync so callers can mark newly created SIP endpoints as internal at creation time.

Why

Plivo CX (Contacto) auto-creates a SIP endpoint per AOM during user invite for browser-SDK playback. These internal endpoints currently leak into the customer's Endpoints tab in the console (Pylon #2460), causing confusion, prod alerts (customers try to register the SIP from third-party clients), and a security concern (customer can silently change creds).

The trace service already supports an internal column on the endpoint model + defaults the List API to exclude internal endpoints (commit cea50cf / d425930 on trace). What's missing is the ability for the auto-create call (Hodor) to actually mark endpoints as internal=true at creation time. This SDK change unblocks that.

Change

• src/Plivo/Resource/Endpoint/EndpointInterface.cs — one new optional parameter on both Create and CreateAsync: bool? isInternal = null.
• When isInternal.HasValue, the value is added directly to the outgoing request dictionary under the wire key internal (lowercase) — bypassing the anonymous-object -> snake_case conversion done by CreateData, which would otherwise emit is_internal.

C# reserved-keyword workaround

internal is a C# keyword and cannot be used as a parameter identifier, so the SDK-facing parameter is isInternal. The value is stamped onto the request body with data["internal"] = isInternal.Value; so the wire contract (internal) still matches the Trace backend and the other-language SDKs.

Backwards-compatibility

Optional nullable parameter, default null. When isInternal is null, the key is omitted from the request body entirely — every existing caller sees identical wire behavior. Only callers that explicitly pass isInternal: true opt into the new path.

Test plan

• dotnet build src/Plivo/Plivo.csproj clean (not runnable in this env — no local dotnet installed; verify in CI)
• Existing Endpoint tests pass
• End-to-end sanity: call Endpoint Create with isInternal: true against api.plivo.com from a test account and verify subscriber.internal = true on the row (validated separately by the CX team's integration)

Related

• JIRA: VT-9534
• Slack: [thread on Pylon #2460]
• Companion PRs across other SDKs: plivo-go version fix #250, plivo-python, plivo-node, plivo-ruby, plivo-java, plivo-php (following this pattern)

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