feat(socrate): ServiceTokenWithScopes, a service token requested with scopes - #125

What and why

(*socrate.Client).ServiceToken requests its client_credentials token without a scope parameter, so Socrate issues its default api. Since Socrate v1.14.0, a client can be granted application-defined scopes (<namespace>:<name>, e.g. swingdrift:worker) in its allowed_scopes, but it has to request them. So a service using backendkit couldn't obtain the scope that a resource server's jwtauth.RequireScopes("swingdrift:worker") (v1.26.0) requires.

New API (additive):

func (c *Client) ServiceTokenWithScopes(ctx context.Context, scopes ...string) (token string, expiresAt time.Time, err error)
var ErrScopeNotGranted = errors.New("socrate: scope not granted")

• Same contract as ServiceToken:
  • cached, and renewed within 30 s of expiry;
  • concurrent callers share one exchange;
  • requires ClientSecret;
  • sends the correlation ID;
  • the token is never logged or put in an error.
• Cache per scope set: duplicates are removed and the set is sorted, so ("a","b") and ("b","a","a") share a token. Each set has its own lock, so different sets don't block each other. At most 16 sets are cached; beyond that the least recently used one is dropped.
• Fails closed without a network call when:
  • no scope is given (use ServiceToken for the default);
  • a value isn't an RFC 6749 §3.3 scope-token;
  • ClientSecret is missing.
• Checks what Socrate granted:
  • invalid_scope (a scope the client doesn't register) matches both ErrScopeNotGranted and *OAuthError.
  • A response whose scope lacks a requested scope is ErrScopeNotGranted, naming the missing scopes.
  • A response without scope means the request was granted as asked (RFC 6749 §5.1).
• Socrate's behaviour, read from its v1.14.1 source: it issues exactly the requested scopes, without adding the default api. The docs tell callers to list api too when they need it.
• ServiceToken is unchanged: it has its own cache and still sends no scope. The token exchange moved into a shared unexported helper with identical error texts. InvalidateServiceToken() now also clears the scoped tokens; existing callers have none.
• Docs: the README socrate section, a "Which package" row and troubleshooting rows. docs/CLIENT-INTEGRATION.md gets the calling side: register the scope in Socrate, then call ServiceTokenWithScopes. The README's "app scopes on every grant" is corrected to "when the client requests them".

How it was tested

socrate/service_token_scopes_test.go (13 tests, httptest standing in for Socrate's /oauth/token) covers:

• the scope parameter is sent normalised;
• the cache is hit for the same set in any order, with separate tokens per set;
• ServiceToken is unaffected;
• renewal near expiry;
• concurrent callers share one exchange (-race);
• an invalid or empty scope list makes no request;
• invalid_scope is recognisable;
• a missing or non-string response scope, and null or an absent scope accepted;
• LRU eviction;
• no token in any error;
• the correlation ID is sent;
• invalidation clears the scoped tokens.

One test added to service_token_test.go pins down that ServiceToken accepts any scope type, as before.

• go mod tidy && git diff --exit-code go.sum leaves go.sum unchanged
• go build ./... passes
• go vet ./... passes, also with -tags conformance
• go test -race -count=1 -timeout=120s ./... passes, also ./socrate/... on the go 1.26.0 floor
• golangci-lint run ./... (v2.14.0) reports no issue
• govulncheck ./...: not run locally (vuln.go.dev is blocked here); CI runs it. No dependency change.
• A line is added under ## [Unreleased] in CHANGELOG.md (### Added)

Compatibility

• Exported-API change: yes, additive: (*Client).ServiceTokenWithScopes, ErrScopeNotGranted.
• Behaviour change for existing callers: InvalidateServiceToken also clears scoped tokens, which they don't have. Nothing else.
• Breaking change: none.

This PR and #124 (the changelog fix) both touch CHANGELOG.md. I'll merge main into whichever merges second.

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