Refactor oauth

[darinkrauss]

• Refactor oauth
• Update TokenSource functions to return whether change occurred
• Add optional client error response parser
• Update Dexcom package accordingly
• Add mocks

[Copilot]

Pull request overview

Refactors OAuth token-source handling to expose whether token state changed, adds optional client-side error parsing, and updates Dexcom/OAuth clients and mocks to match.

Changes:

• Update TokenSource APIs (UpdateToken/ExpireToken) to return (bool, error) and propagate conditional session updates.
• Add NewWithErrorParser constructors to OAuth client/provider to support optional error-response parsing.
• Update tests and add generated GoMock mocks for OAuth interfaces.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
private/plugin/abbott	Bumps the Abbott submodule reference.
oauth/work/mixin.go	Updates token update/expire flow to return change flags and handle missing token source.
oauth/token/source.go	Implements (bool, error) semantics for token refresh/expire and adds “no-op” behavior.
oauth/test/token_source.go	Updates the hand-rolled test stub to support (bool, error) outputs.
oauth/test/oauth_mocks.go	Adds generated GoMock mocks for OAuth interfaces.
oauth/provider/client/client.go	Adds provider constructor supporting optional error parser and wires it into oauth client creation.
oauth/oauth.go	Updates TokenSource interface signatures and adds go:generate directives for mocks.
oauth/client/client.go	Adds client constructor supporting optional error parser; adapts to new token-source signatures.
oauth/client/client_test.go	Updates tests to use the new (bool, error) token source outputs.
dexcom/fetch/runner.go	Updates Dexcom runner token handling to return change flags and conditionally persist session updates.
dexcom/client/client_test.go	Updates Dexcom client tests to use the new token source outputs.

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[darinkrauss]

Updates based upon feedback included in later PR.