Fix testifylint violation: use assert.Len for collection length checks

[Copilot]

CI failing on testifylint rule violation in pkg/workflow/http_mcp_env_vars_test.go:70.

Change

Replace assert.Equal with assert.Len for checking collection length:

// Before
assert.Equal(t, 3, len(envVars), "Should have exactly 3 environment variables")

// After  
assert.Len(t, envVars, 3, "Should have exactly 3 environment variables")

The assert.Len helper provides better failure messages and is the idiomatic way to assert collection lengths in testify.

Original prompt

Reference: https://github.com/githubnext/gh-aw/actions/runs/21197211966/job/60975657682#step:7:1

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