feat: upsert team members by email

[despreston]

Summary

• Add an opt-in adopt_existing argument to launchdarkly_team_member.
• When LaunchDarkly returns email_already_exists_in_account, look up the member by email, set the Terraform state ID, and run the existing update/read flow for mutable fields.
• Document that adopted members can have role/custom role/role attribute fields reconciled, but first and last names cannot be updated by admins.

Why

Many organizations gradually move LaunchDarkly account membership into Terraform after users have already been invited manually, through SSO, or by another automation path. In those setups, a Terraform apply that tries to grant or reconcile access for an existing member currently fails at create time because the email address already exists in the account. This option lets teams explicitly choose to adopt that existing member into state and continue reconciling the fields Terraform can manage, without changing the default create-only behavior for users who want strict ownership semantics.

Testing

• GOFLAGS='' PATH=/home/ubuntu/co/backend/opt/go1.24.2/bin:$PATH make fmtcheck
• GOFLAGS='' PATH=/home/ubuntu/co/backend/opt/go1.24.2/bin:$PATH /home/ubuntu/co/backend/opt/go1.24.2/bin/go test ./launchdarkly -run '^$'
• GOFLAGS='' PATH=/home/ubuntu/co/backend/opt/go1.24.2/bin:$PATH /home/ubuntu/co/backend/opt/go1.24.2/bin/go test ./launchdarkly -run '^TestAccTeamMember_AdoptExisting$' -count=1 (skips locally without TF_ACC)

I also ran go test ./launchdarkly -run 'Test[^A]'; it compiles but currently fails on unrelated tests on this checkout: Test404RetryClient, TestEnvironmentToResourceData, TestValidateRuleResourceData, TestViewRequestsIncludeUserAgentHeader, and team maintainer fixture tests.