feat(sts): send RFC 8707 resource and audience on token exchange

[QuentinBisson]

What

The Go ADK token-propagation plugin (go/adk/pkg/sts) always passed nil
resource and nil audience to the STS token exchange, so an issued token
could not be scoped to a specific backend. This wires two optional inputs
through to the exchange:

• KAGENT_STS_RESOURCE: RFC 8707 resource indicator (the target backend).
• KAGENT_STS_AUDIENCE: RFC 8693 audience, for STS servers that key on it.

Both are read at agent-runtime startup and passed as resource/audience
parameters to NewTokenPropagationPlugin. Empty values are omitted from the
request, so behaviour is unchanged when neither is set.

Why

Without a resource/audience, the STS returns a token whose audience is not the
MCP backend, so audience-validating backends reject it. RFC 8707 is the
standard way to scope an exchanged token to its intended resource. The STS
client already serialized resource/audience; only the call site and the
configuration plumbing were missing.

Notes

• Backwards compatible: no env set means no resource/audience sent.
• The env vars are registered in go/core/pkg/env for discoverability. Moving
  this into the adk config file (with a first-class Agent CRD field) is under
  discussion; see the thread below.

[QuentinBisson]

Python companion: #2107 (same env vars across both runtimes).

[Copilot]

Pull request overview

This PR wires optional RFC 8707 resource and RFC 8693 audience parameters through the Go ADK STS token-exchange flow so exchanged tokens can be scoped to the intended backend (configured via new KAGENT_TOKEN_RESOURCE / KAGENT_TOKEN_AUDIENCE env vars).

Changes:

• Register KAGENT_TOKEN_RESOURCE and KAGENT_TOKEN_AUDIENCE in the core env registry for discoverability.
• Add an options pattern to the STS token propagation plugin and plumb resource/audience into ExchangeTokenWithActorToken.
• Read env vars at agent runtime startup and add a test asserting the form params are sent.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
go/core/pkg/env/kagent.go	Registers env vars for token-exchange resource/audience.
go/adk/pkg/sts/plugin.go	Adds WithExchangeTarget option and passes resource/audience into STS exchange calls.
go/adk/pkg/sts/plugin_test.go	Adds a test verifying resource/audience are present/absent on the exchange request.
go/adk/pkg/runner/adapter.go	Reads env vars and configures the STS plugin with WithExchangeTarget.

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[QuentinBisson]

Thanks — addressed in 7dccee6:

• any exchange target (plugin.go): WithExchangeTarget now takes string instead of any, so a caller can't pass a type the STS client silently drops; empty values are omitted (unset → unscoped, as before). Multi-valued resources are out of scope for this change.
• Test data race (plugin_test.go): the handler now sends the captured form over a buffered channel; the test reads it on its own goroutine, so there's no shared-variable race and no t.Fatalf from the handler goroutine. Verified with go test -race.
• Hang safety: the assertion waits on the channel with a 5s timeout instead of reading a variable that may never have been written.

[EItanya]

Why are these any, they can be string or []string?

[QuentinBisson]

I updated them to []string to be spec compliant :)

[EItanya]

Frankly I don't really like adding more env vars if we don't have to, they're hard to track. Can we add a token exchange section to our adk config file instead? This is for the Python impl as well

[QuentinBisson]

Thanks for the review. I'll see what I can do :)

[QuentinBisson]

@EItanya thanks for the review. Addressed the two smaller points and pushed:

• Dropped the functional option: NewTokenPropagationPlugin now takes resource, audience string directly and the fields are typed []string.
• Renamed the env vars to KAGENT_STS_RESOURCE / KAGENT_STS_AUDIENCE here and in feat(sts): send RFC 8707 resource and audience on token exchange (Python) #2107.

On moving these into the adk config file instead of env vars: happy to. Before I wire it, one scoping question so I size the PRs right. My plan is a tokenExchange: { resource, audience } section in the agent config.json both runtimes already read, populated by a new Agent CRD field the controller renders, with the env vars dropped. Do you want that CRD field plus rendering inside these two PRs, or should these ship env-based as-is and the config section land as a follow-up (Go and Python together)?