k8s.io/apimachinery v0.36.2 raised the go.mod go directive to 1.26.0,
but CI pinned GO_VERSION=1.25.10 with GOTOOLCHAIN=local, causing lint,
unit-test and build jobs to fail with:
  go: go.mod requires go >= 1.26.0 (running go 1.25.10; GOTOOLCHAIN=local)

The golangci-lint v2.12.2 bump (needed for Go 1.26) surfaced goconst
findings that v2.8.0 did not report:

- fn.go: extract repeated Microsoft Graph field-name literals
  (displayName, appId, description) into named constants.
- .golangci.yml: set goconst.ignore-tests to skip repeated string
  literals in test fixtures, where extracting constants would only
  reduce readability.

No behavior change. Verified locally with golangci-lint v2.11.4
(built with go1.26.1): 0 issues, all unit tests pass.

Reverts goconst.ignore-tests so test code is linted too, and instead
resolves the goconst findings by extracting the repeated string literals
in fn_test.go into named constants:

- fn.go: add fieldMail, fieldUserPrincipalName, fieldType field-name
  constants (alongside the existing displayName/appId/description) and
  use them in production map keys and $select projections.
- fn_test.go: add a fixture const block (testRequestTag, testAzureCredsName,
  condTypeFunctionSuccess, the per-query-type messages, etc.) and reuse
  the field-name constants across the table-driven cases.

Literals inside raw-string JSON fixtures are left untouched (they are
single string literals, not separate tokens). No behavior change;
verified locally with golangci-lint: 0 issues, all unit tests pass.

Two goconst findings surfaced only under golangci-lint v2.12.2 (local
v2.11.4 did not report them):

- Promote userType/servicePrincipalType/unknownType from function-local
  constants in processMember to package level, and reuse them for the
  repeated "user"/"servicePrincipal" literals in fn_test.go.
- Add testUser1Email for the repeated "user1@example.com" fixture.

Verified with golangci-lint v2.12.2 (CI parity): 0 issues, tests pass.