feat(symbols): derive the upload endpoint from --base-uri

[abelonogov-ld]

Summary

Uploading symbols to anything other than production took two flags that had to agree: --base-uri to name the instance, and --backend-url to name that same instance's observability API. Staging looked like this:

ldcli symbols upload --type android --project my-project \
  --base-uri https://ld-stg.launchdarkly.com \
  --backend-url https://pri.observability.ld-stg.launchdarkly.com \
  --access-token api-...

The second flag is derivable from the first. Every instance publishes its observability API under a host named for the instance, which is exactly how the production default in this file was already built, so --backend-url now defaults to the API of whichever instance --base-uri names and the command above loses a line.

Only LaunchDarkly's own hosts are derived from. A base URI aimed at a local stack or a proxy says nothing about where its observability API listens, so those keep today's production default, and --backend-url still overrides everything — which is how local development already points at http://localhost:8082/private.

Behavior for the default base URI is unchanged: app.launchdarkly.com derives the same pri.observability.app.launchdarkly.com the constant held. The flag's registered default moves from that constant to empty so the derivation can run; --help now names the production URL in the description instead.

Test plan

• go test ./cmd/symbols/ — new TestDefaultBackendURLFor covers production, staging, a regional host, trailing slashes and whitespace, and the cases that fall back to the default (local stack, a host that merely ends in the domain name, unset, unparseable)
• go build ./...
• Upload Android symbols to staging with only --base-uri https://ld-stg.launchdarkly.com and confirm they land

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