Add a new `infrastructure` package that centralises loading of the
infrastructure config and instantiation of the secrets provider, replacing the
per-package loading logic that previously lived in `postgres`.

`infrastructure.DefaultConfig()` lazily loads `/etc/labkit/infrastructure.yaml`
and a file-backed secret provider rooted at `/secrets/infrastructure`, caching
the result for subsequent callers.

Refactor the PostgreSQL client to consume `*infrastructure.Config` via a new
`WithInfrastructure` option, which is primarily useful for testing. When no
config is provided, `postgres.New(ctx)` falls back to `DefaultConfig()`, so the
developer-friendly zero-argument API is preserved and LabKit will do the right
thing.

As part of the refactor:

- Replace `ErrUndefinedDependency` with the more specific `ErrMissingConfig`
  and `ErrMissingSecrets`.
- Move secret lookups from a postgres-specific mount (`/secrets/infrastructure/postgresql`)
  to keyed lookups (`postgresql/username`, `postgresql/password`) against the
  shared provider rooted at `/secrets/infrastructure`.
- Rewrite `postgres` tests to construct `*infrastructure.Config` directly with
  a `secret.MemoryProvider`, avoiding filesystem fixtures and adding coverage
  for missing-secret error paths.

Issue: gitlab-com/gl-infra/platform/runway/team#884

Infrastructure clients should return this error if the client was not
configured, giving callers a consistent way to check for this condition.

Document error propagation behaviour and update Config godoc

See merge request !487

Merged-by: Matias Alvarez <malvarez@gitlab.com>
Approved-by: Luke Hollinda <lhollinda@gitlab.com>
Approved-by: Matias Alvarez <malvarez@gitlab.com>
Co-authored-by: Elliot Forbes <eforbes@gitlab.com>

Adds two integration tests using a hung httptest.Server to verify that
evaluation completes within the configured timeout, returns the default
flag value, propagates an observable error, and records the failure on
the OTel span. Closes criterion 5 of team-tasks#4382.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

test(v2/featureflag): add integration tests for timeout fallback behaviour

See merge request !495

Merged-by: Matias Alvarez <malvarez@gitlab.com>
Approved-by: Matias Alvarez <malvarez@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: e_forbes <eforbes@gitlab.com>

feat(infrastructure): Introduce shared infrastructure config package.

See merge request !467

Merged-by: Elliot Forbes <eforbes@gitlab.com>
Approved-by: Elliot Forbes <eforbes@gitlab.com>
Reviewed-by: GitLab Duo <gitlab-duo@gitlab.com>
Co-authored-by: Florian Forster <fforster@gitlab.com>