feat(mcp): #33 add is_rejected_with_code and because_message_equals

[othercodes]

Closes #33.

Summary

Two thin additions to AssertableMCP that close gaps in the fluent error vocabulary:

• is_rejected_with_code(code: int) — assert arbitrary JSON-RPC error codes (custom application-level codes outside the spec set, e.g. -32001 for auth failures).
• because_message_equals(expected: str) — exact match sibling of the existing because_message_contains(substr).

Both compose inside assert_mcp(lambda m: ...), eliminating the need to drop into assert_json("error", ...) for custom error codes or strict message matching.

Before / after

# Before — bypass MCP module for custom codes
response.assert_unauthorized().assert_json(
    "error",
    lambda e: e.where("code", -32001).where("message", AUTH_ERROR_MESSAGE),
)

# After — stays in MCP fluent style
response.assert_unauthorized().assert_mcp(lambda m:
    m.is_rejected_with_code(-32001).because_message_equals(AUTH_ERROR_MESSAGE)
)

Implementation

• Widened _expect_error_code signature from ErrorCode to int | ErrorCode (no behavior change — internal already used int(expected)).
• is_rejected_with_code is a 2-line public wrapper over the internal helper.
• because_message_equals mirrors because_message_contains with == and consistent error-message wording ("Error message does not match X: Y").

Test plan

• is_rejected_with_code happy + sad path + "no error envelope" branch.
• because_message_equals happy + sad path + "no error envelope" branch.
• mypy strict clean.
• Coverage 100% maintained.
• ruff lint + format clean.
• README MCP method catalog updated.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud