Telegram: support custom Bot API base URL

[AyobamiH]

What

Add support for an optional custom Telegram Bot API base URL for Grafana-managed alerting.

When api_url is configured, Telegram requests are sent to:

<api_url>/bot<token>/<action>

When it is omitted, the notifier continues using the existing Telegram API endpoint.

Why

This enables Grafana alerting to work with local, proxied, or self-hosted Telegram Bot API servers without changing the existing behavior for users of the public Telegram API.

Fixes grafana/grafana#81534.

Implementation

• Adds optional api_url support to the Telegram v1 configuration and schema.
• Validates that configured values are absolute HTTP(S) URLs.
• Normalizes trailing slashes before request construction.
• Preserves the existing APIURL integration-test override when no custom URL is configured.
• Routes both message and image actions through the configured base URL.
• Updates the complete Telegram configuration fixture.

Verification

Regression coverage includes:

• custom Bot API routing
• existing default routing
• trailing-slash normalization
• rejection of relative URLs
• rejection of non-HTTP(S) URLs
• full Telegram configuration parsing

Fork CI on the submitted commit passes:

• make mod-check
• make lint
• make test
• TruffleHog secret scanning

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