Fix TLS authority case sensitivity and kctrl error message grammar/formatting

[sameerforge]

What this PR does / why we need it:

DNS/registry hostnames are case-insensitive per RFC 1035, but two comparisons in this codebase treated them as case-sensitive:

• Config.ShouldSkipTLSForAuthority (pkg/config/config.go) compared the admin-configured dangerousSkipTLSVerify allow-list against the host parsed from an App/PackageRepository's fetch URL using ==. Since the allow-list and the fetch URL are typed independently by different people (a cluster admin editing the kapp-controller-config Secret/ConfigMap vs. an app author writing a CR), a casing difference between the two (e.g. Registry.Example.Com vs. registry.example.com) silently left DangerousSkipTLSVerify false. The fetch then attempted real TLS verification against a host the admin explicitly intended to skip, failing with an x509: certificate signed by unknown authority error.
• kctrl package repository add/update (cli/pkg/kctrl/cmd/package/repository/add_or_update.go) compared the --url flag against the already-persisted PackageRepository's stored image ref with ==. A registry host typed with different casing than what's already stored missed the no-op/idempotency short-circuit, forcing an unnecessary Update() call instead of just tailing status.

Both comparisons now use strings.EqualFold instead of ==. Added mixed-case regression cases to Test_ShouldSkipTLSForAuthority covering both the bare-host and host:port branches.

Additionally, this PR includes a set of grammar and formatting fixes to kctrl CLI error and help messages, found via a targeted error-message audit:

• NamespaceNameFlag.resolveValue (cli/pkg/kctrl/cmd/core/namespace_flags.go): fixed the malformed error text "Expected to non-empty namespace name" → "Expected a non-empty namespace name".
• SecureNamespaceFlags.CheckForDisallowedSharedNamespaces (cli/pkg/kctrl/cmd/core/secure_namespace_flags.go): added missing spaces between concatenated string segments, which previously ran words together (e.g. namespace (%s)(hint: ...).
• kctrl completion (cli/pkg/kctrl/cmd/completion.go): the command's Short/Long help text and its default case error only listed bash, zsh or fish despite powershell being a supported and documented (ValidArgs, Example) shell; both now list all four shells, and the error message is capitalized consistently with other kctrl errors.
• PackageValuesSchemaParser.walkOnValueSchemaProperties (cli/pkg/kctrl/cmd/package/available/values_schema.go): fixed a missing space in the concatenated error string ("...has unsupported"+" type %v...") and capitalized the message for consistency.
• Standardized the inconsistent adjective "non empty" to the correct compound adjective "non-empty" across 8 kctrl command files (app/delete.go, app/get.go, app/kick.go, app/pause.go, app/status.go, package/repository/kick.go, package/installed/{status,delete,create_or_update,get,pause_or_kick}.go).

None of these are functional changes — they only affect the literal text of user-facing CLI error/help strings.

Which issue(s) this PR fixes:

Fixes #1864

Does this PR introduce a user-facing change?

Fixed two case-sensitivity bugs where a hostname/registry reference with different casing than a previously-recorded value was incorrectly treated as a mismatch: `dangerousSkipTLSVerify` allow-list entries now match fetch URL hosts regardless of case, and `kctrl package repository add/update` now correctly recognizes a no-op update when only the registry host's casing differs. Also fixed several grammar, spacing, and consistency issues in `kctrl` CLI error and help messages.

[joaopapereira]

Looks good in general 1 issues that I think we should correct.
Another thing is Release note is accurate; optional: file a tracking issue (body invites that).

[joaopapereira]

Only the registry host is case-insensitive; org/repo path and tag can be case-sensitive (e.g. GHCR). Folding the whole string can treat distinct refs as a no-op. Prefer parse with name.ParseReference (already imported) and fold only the registry, or compare RegistryStr() case-insensitively and the remainder exactly.

[sameerforge]

I've updated the implementation to use name.ParseReference() and now compare only the registry case-insensitively via Context().RegistryStr(), while comparing the repository path and tag case-sensitively. This prevents false matches when refs differ only in case.

[sameerforge]

@joaopapereira Thanks for the review! I've addressed the case-sensitivity issue with proper name.ParseReference() parsing, comparing only the registry hostname case-insensitively while treating the repository path and tag case-sensitively. Filed tracking issue #1864 and updated the PR description. All CI checks pass — Ready for your approval now.

[joaopapereira]

Lgtm