feat(server): add CONNECTOR_DISCOVERY_ENABLED flag to disable registry polling

[antoniocali]

Summary

Closes part of #380.

Adds a new server config flag CONNECTOR_DISCOVERY_ENABLED (default true) that, when disabled, short-circuits utils.GetDriverImageTags and returns a configurable DEFAULT_CONNECTOR_VERSION instead of querying Docker Hub / ECR / GCR Artifact Registry. Default-on preserves the current behavior for every existing deployment.

This addresses the Fusion-only / air-gapped use case described in #380, where the UI hits Docker Hub on every /versions, /spec, and /test-connection call even though CDC connectors are unused. With discovery disabled, those endpoints return the configured version without an outbound HTTP request, eliminating both the user-facing slowness during rate-limited periods and the log noise.

This is suggestion (1) from the issue. Suggestion (5) (rewording the K8s warning) is bundled here since it's a one-liner and naturally points operators at the new flag. Suggestions (2) backoff, (3) DB spec cache, and (4) custom discovery registry are intentionally out of scope.

Changes

• server/internal/appconfig/appconfig.go — two new fields on Config: ConnectorDiscoveryEnabled (bool) and DefaultConnectorVersion (string).
• server/conf/app.yaml — defaults: CONNECTOR_DISCOVERY_ENABLED: true, DEFAULT_CONNECTOR_VERSION: "". Behavior unchanged unless an operator explicitly opts in.
• server/internal/utils/docker_utils.go — GetDriverImageTags short-circuits at the top of the function when discovery is disabled; returns (nil, "", error) with a clear message if DEFAULT_CONNECTOR_VERSION is unset.
• server/internal/utils/docker_utils.go — rewords the Kubernetes "Cached fallback unavailable on Kubernetes (no Docker daemon)" warning at line 103 to be less alarming and to mention the new flag.

Helm chart

A separate docs-only PR to datazip-inc/olake-helm will surface the flag via olakeUI.env examples + a README note. No template change is required there because olakeUI.env is already a passthrough map.

Operator usage

# values.yaml
olakeUI:
  env:
    CONNECTOR_DISCOVERY_ENABLED: "false"
    DEFAULT_CONNECTOR_VERSION: "v0.3.18"

Test plan

• go build ./... clean
• go vet ./... clean
• Manual: verify default (CONNECTOR_DISCOVERY_ENABLED=true) preserves existing /versions and /spec behavior
• Manual: verify CONNECTOR_DISCOVERY_ENABLED=false with DEFAULT_CONNECTOR_VERSION=v0.3.18 makes /versions return ["v0.3.18"] without any outbound HTTP
• Manual: verify CONNECTOR_DISCOVERY_ENABLED=false with empty DEFAULT_CONNECTOR_VERSION returns a clear error on /versions and /spec

[CLAassistant]

All committers have signed the CLA.

[nayanj98]

@antoniocali we are looking into this issue and your PR. Will have an update on it soon.