[TF-27883] Add agent execution mode and pool to test config

[hashimoon]

Description

Introduces 'agent_execution_mode' and 'agent_pool_id' fields to the registry module test_config schema, supporting both agent and remote execution modes. Adds validation to prevent agent_pool_id from being set when execution mode is 'remote', updates create, update, and read logic, and provides comprehensive acceptance tests for new behaviors. Also updates dependencies to latest versions.

Remember to:

• Update the Change Log
• Update the Documentation

Testing plan

1. Create a registry module with agent execution mode:
   - Apply the first Terraform config above
   - Verify in TFE/TFC UI that the module's test configuration shows agent execution mode
   - Verify the specified agent pool is being used
2. Update from remote to agent execution mode:
   - Start with a module using remote execution
   - Update the configuration to use agent execution mode with an agent pool
   - Run terraform apply and verify the change is applied successfully
3. Clear agent pool configuration:
   - Start with a module using agent execution mode
   - Change agent_execution_mode back to "remote" and remove agent_pool_id
   - Verify the module reverts to remote execution
4. Validation testing:
   - Attempt to set agent_pool_id without setting agent_execution_mode = "agent"
   - Verify that Terraform returns the appropriate validation error

External links

Include any links here that might be helpful for people reviewing your PR. If there are none, feel free to delete this section.

• go-tfe documentation
• API documentation
• Related PR

Output from acceptance tests

Please run applicable acceptance tests locally and include the output here. See testing.md to learn how to run acceptance tests.

If you are an external contributor, your contribution(s) will first be reviewed before running them against the project's CI pipeline.

go test -v ./internal/provider -run "TestAccTFERegistryModule_agentExecutionModeWithAgentPool|TestAccTFERegistryModule_remoteExecutionModeWithoutAgentPool|TestAccTFERegistryModule_agentPoolFieldClearing|TestAccTFERegistryModule_remoteExecutionModeWithAgentPoolError"
=== RUN   TestAccTFERegistryModule_agentExecutionModeWithAgentPool
--- PASS: TestAccTFERegistryModule_agentExecutionModeWithAgentPool (6.03s)
=== RUN   TestAccTFERegistryModule_remoteExecutionModeWithoutAgentPool
--- PASS: TestAccTFERegistryModule_remoteExecutionModeWithoutAgentPool (5.48s)
=== RUN   TestAccTFERegistryModule_agentPoolFieldClearing
--- PASS: TestAccTFERegistryModule_agentPoolFieldClearing (5.41s)
=== RUN   TestAccTFERegistryModule_remoteExecutionModeWithAgentPoolError
--- PASS: TestAccTFERegistryModule_remoteExecutionModeWithAgentPoolError (0.22s)
PASS
ok  	github.com/hashicorp/terraform-provider-tfe/internal/provider	17.628s
...

Rollback Plan

Changes to Security Controls

[ctrombley]

Looks great, thank you @hashimoon!