Stop nil VCR filters flaking replays, and spell the MCP plural

[TonsOfFun]

Two fixes to the same family of papercut. Both surfaced while diagnosing CI failures on #432, #434 and #435.

1. VCR filters registered against unset env vars

Every filter_sensitive_data block read an ENV var directly:

config.filter_sensitive_data("ACCESS_TOKEN") { ENV["OPEN_AI_ACCESS_TOKEN"] }

CI sets none of them — cassettes replay without credentials — so on CI every block returns nil. VCR registers the filter anyway and then substitutes an empty string through every request and response it handles. That surfaces far from the cause as an intermittent:

NoMethodError: undefined method `each_key' for nil
  webmock-3.26.4/lib/webmock/util/hash_validator.rb:12:in `validate_keys'
  webmock/response.rb:80:in `options='

…while WebMock builds a stubbed response. It reads as a flaky cassette rather than a configuration problem. It failed test (3.3, rails8) on #435 (passed on re-run) and cost an investigation on #432 to rule out.

A filter_env helper now registers a filter only when the variable actually holds something. Replay is deterministic on CI; redaction still applies when recording with real keys.

This also generalises the one-off if ENV["AZURE_OPENAI_RESOURCE"] guard that already existed — the right instinct, applied to exactly one of ten filters.

2. The MCP plural did not inflect

An acronym only matches a whole word, so inflect.acronym "MCP" alone does not give the plural:

input	before	after
mcps.camelize	Mcps	MCPs
mcps_examples_test.camelize	McpsExamplesTest	MCPsExamplesTest
MCPsExamplesTest.underscore	mc_ps_examples_test ❌	mcps_examples_test ✅

That last row is the real bug: a constant spelled MCPs underscored back to a name no file has. Registering the plural as its own acronym makes both directions agree, and the two test classes spelling it Mcps are renamed to match.

Deliberately unchanged: mcps: stays lowercase as a prompt parameter, and Mcp-Session-Id keeps its spelling — that is what the MCP specification names the header. ToolChoiceMcp is an external OpenAI SDK constant.

The engine is unaffected by the new acronyms: action_agent.inflections already isolates every path under the engine root from host-registered acronyms, precisely so a host declaring "API" or "MCP" cannot break its constant lookups.

Verification

• actionagent/test: 340 runs, 0 failures
• The two renamed docs test files match their pre-change baseline exactly (15 runs, 9 errors both before and after — local missing-credential errors, since this machine has no .env.test)
• RuboCop clean

🤖 Generated with Claude Code

https://claude.ai/code/session_01He34kWksjpqPPDpCCqJq2C