fix(analytics): classify opencode-go as a flat-rate subscription

[pacocartones]

opencode-go is a flat monthly subscription (https://opencode.ai/go) but is missing from FLAT_RATE_SUBSCRIPTION_PROVIDER_IDS, so cost analytics prices it per token against the underlying models' metered rates. The dollar figure shown does not correspond to anything the user is billed.

Root cause

src/lib/usage/flatRateProviders.ts keeps an explicit set of subscription/coding-plan ids. opencode-go is absent from it, and it is api-key auth (registered in shared/constants/providers/apikey/gateways.ts:613), so it is not covered by the dynamic WEB_COOKIE_PROVIDERS branch either. isFlatRateProvider("opencode-go") therefore returns false.

Everything else in the path already works: every analytics surface passes flatRateAsZero, so the single missing id is the whole fault. This follows the exact pattern of the claude/cc addition in #10774.

Why the overstatement is large

opencode-go is an aggregator, so one flat subscription resells GLM, Kimi, Grok, DeepSeek, MiniMax, Qwen and GPT-5.x, and each call is priced at the underlying model's rate rather than a single cheap rate. The reporter measured $13.35 reported over 30 days against a $10 flat actual bill.

Red / green

Before, on a clean checkout at 62ab93d78:

✖ isFlatRateProvider: dedicated subscription / coding-plan providers are flat-rate
  AssertionError: opencode-go should be flat-rate
  false !== true
✖ #11149: opencode-go is a flat-rate subscription, not metered
  false !== true
ℹ pass 8  ℹ fail 2

After:

✔ #11149: opencode-go is a flat-rate subscription, not metered
✔ #11149: sibling opencode ids keep their own billing semantics
ℹ tests 10  ℹ pass 10  ℹ fail 0

The new cases extend the existing flat-rate-cost-5552 suite rather than adding a file. They assert the zeroing is still opt-in (without flatRateAsZero the estimate is unchanged, so budget/quota/routing paths are untouched) and that the keyless opencode id is not swept in by a prefix-style match.

Per the note in the issue, this reads as an oversight rather than a deliberate exclusion, so the id is added to the set with a comment explaining the aggregator reasoning, keeping the documented "deliberately excluded" list accurate.

@diegosouzapw one CI note: release/v3.8.50 is red at the base right now (stale ESLint suppressions, unit shards, Fast Quality Gates), so those reds should show on other PRs too and are not from this change.

Closes #11149