feat: add fail-now checker

[mmorel-35]

assert.Fail, assert.FailNow (and their f/require variants) are thin wrappers over standard testing.T methods. This checker detects their use and auto-fixes to the direct stdlib equivalents.

Checker: fail-now

• Enabled by default: true
• Autofix: true (when the call result is discarded and the target method exists on the t type)

Fix mapping

Detected	Fixed
assert.Fail(t, "msg")	t.Error("msg")
assert.Fail(t, "fail", "msg")	t.Error("msg")
assert.Fail(t, "fail", "fmt %s", arg)	t.Errorf("fmt %s", arg)
assert.Failf(t, "fail", "fmt %s", arg)	t.Errorf("fmt %s", arg)
assert.FailNow(t, "msg")	t.Fatal("msg")
assert.FailNow(t, "fail", "fmt %s", arg)	t.Fatalf("fmt %s", arg)
assert.FailNowf(t, "fail", "fmt %s", arg)	t.Fatalf("fmt %s", arg)
s.FailNow("msg") (suite receiver)	s.T().Fatal("msg")

When msgAndArgs are present in non-f variants, failureMessage is dropped and msgAndArgs become the replacement arguments (matching testify's own messageFromMsgAndArgs convention). Suite method calls are fixed via receiver.T() when the receiver implements TestifySuite.

Fix safety constraints

• Expression context: the autofix is only emitted when the call result is definitely discarded (ExprStmt/GoStmt/DeferStmt). In expression contexts (e.g. _ = assert.Fail(...), if assert.Fail(...) {}), the diagnostic is reported but no fix is suggested, to avoid producing uncompilable code.
• Method availability: the fix is gated on the static type of the t argument having the target method (checked via types.LookupFieldOrMethod). If t only implements assert.TestingT (which only requires Errorf), the non-f fix is withheld.
• Assertion objects: calls on non-suite method receivers (e.g. assertObj.Fail(...) where assertObj is *assert.Assertions) are ignored entirely — no diagnostic is emitted since there is no accessible testing.T to suggest.

Implementation

• internal/checkers/fail_now.go — checker (implemented as AdvancedChecker using insp.WithStack for parent-node context) + fix logic
• internal/checkers/checkers_registry.go — registered in the advanced-checker section, enabled by default
• internal/testgen/gen_fail_now.go — test generator with errored + golden templates covering all arg variants, suite method calls, expression-context no-fix cases, and ignored assertion-object calls
• analyzer/testdata/src/go-require-http-handlers/ — updated test + golden to reflect that assert.FailNow and s.FailNow in HTTP handlers are now also fixed by this checker

Closes #144