feat(stream): expose OTLP base URL on source API response (Phase 3)

[jordanrburger]

Stacked on top of #2595. Targets feat/stream-otlp-logs-phase1; rebase to main after that merges.

Summary

Surfaces the OTLP/HTTP endpoint URL alongside the existing stream URL on every source API response so the UI can display it without reconstructing it client-side. Closes the "Phase 3" backend scope from the design doc — the remaining Phase 3 items (one-click UI button, column-mapping JSON templates, quickstart docs) live in keboola-ui and the docs repos, not here.

Changes

• Source.FormatOTLPSourceURL(publicURL) — mirrors FormatHTTPSourceURL but with the /otlp/ path prefix. Returns the base URL only; the OTel SDK appends /v1/logs, /v1/metrics, /v1/traces itself when given OTEL_EXPORTER_OTLP_ENDPOINT.
• api.HTTPSource.OtlpURL field in the Goa design — regenerated server stubs, OpenAPI v3 / v2 JSON / YAML.
• mapper.NewSourceResponse and mapper.NewAggregationSourceResponse populate the new field.
• 15 E2E response fixtures updated to include otlpUrl alongside url inside their http blocks.

Example

$ curl https://stream.keboola.com/v1/branches/123/sources/my-source
{
  ...
  \"http\": {
    \"url\":     \"https://stream-in.keboola.com/stream/12345/my-source/abc...\",
    \"otlpUrl\": \"https://stream-in.keboola.com/otlp/12345/my-source/abc...\"
  },
  ...
}

User then sets one env var and any OTel SDK works:

export OTEL_EXPORTER_OTLP_ENDPOINT=https://stream-in.keboola.com/otlp/12345/my-source/abc...

Test plan

• go build ./internal/pkg/service/stream/... passes
• go vet clean
• Unit test for FormatOTLPSourceURL (canonical case, trailing-slash normalization, invalid-URL error path)
• E2E tests against API in CI (existing test suite — fixtures updated, mapper change wired through)
• Manual smoke: create a source via API, confirm both fields appear in the response

Out of scope

• One-click "Create OTel Source" wizard in the Keboola UI (separate repo)
• Pre-built column-mapping JSON templates for otel_logs / otel_metrics / otel_traces (no Stream-side template system to land them in; better delivered as docs or UI fixtures)
• Quickstart guide / column mapping reference docs (docs repo)

🤖 Generated with Claude Code

[Matovidlo]

closing in favor of #2598