chore(utilities): add tests for previously-uncovered helpers

[mastermanas805]

What

Adds unit tests for three small files in internal/utilities/ that had zero coverage:

• strings.go — StringValue and StringPtr
• context.go — contextKey.String, WithRequestID, GetRequestID, WaitForCleanup
• io.go — SafeClose

Why

Coverage of internal/utilities/ was the lowest among utility-style packages in the module:

internal/utilities                   50.8%
internal/utilities/siwe              96.2%
internal/utilities/siws              97.2%
internal/utilities/version          100.0%
internal/crypto                      91.2%
internal/conf                        99.1%

Each of the helpers covered by this PR is a pure-ish function used widely across the API layer, so a behavior change here would propagate quickly. Locking the contracts in tests makes the next maintenance pass safer without changing any production code.

Coverage delta

Package	Before	After
internal/utilities	50.8%	62.4%

(+11.6 percentage points, no production code touched.)

What's tested

strings.go

• StringValue(nil) and StringValue(&"") both return ""
• StringValue(&s) returns s for non-empty s
• StringPtr("") returns nil; StringPtr(s) returns *s for non-empty
• The pointer returned by StringPtr is independent of the input variable (defends against future refactors that take the address of a closure-captured variable)
• StringValue(StringPtr(c)) == c round-trip across edge cases (empty, ASCII, whitespace, newline, multi-byte rune)

context.go

• contextKey.String() formatting (covers both populated and empty key)
• WithRequestID / GetRequestID round-trip
• GetRequestID returns "" when key absent
• Replacing a previously-set request ID
• Inheritance through context.WithCancel (defends the public-API guarantee that derived contexts see the parent value)
• WaitForCleanup returns when the wait group completes
• WaitForCleanup returns when the context is cancelled, even if the wait group never completes (covers the leak path)

io.go

• SafeClose invokes Close() on the underlying closer
• SafeClose does not panic when Close() returns an error
• SafeClose does not panic on io.NopCloser(nil)

How to verify (for reviewers)

go test ./internal/utilities/ -count=1 -v -run 'TestStringValue|TestStringPtr|TestContextKey|TestRequestID|TestWaitForCleanup|TestSafeClose|TestStringValueAndStringPtr'
go test -cover ./internal/utilities/

The -cover line should report 62.4%.

Verification I ran locally

• go test ./internal/utilities/ -count=1 -v — all new tests pass
• go test ./... -count=1 -p 1 -race — full module suite green
• gofmt -l on the three new files — empty
• go vet ./... — empty

Notes

• No production source files were touched; this is purely a test addition.
• Naming follows the existing repo convention (<file>_test.go per source file).
• WaitForCleanup is exercised end-to-end via real goroutines plus context.WithCancel, matching how the production callers use it. No mocking of channels.
• I deliberately stopped at "small simple changes" scope: the package's other low-coverage areas (postgres.go, hibpcache.go, request.go::GetBodyBytes) need real Postgres errors / bloom-filter setup / multipart bodies and would each be its own PR.