Configured picker treats empty provider headers as configured

[Hmbown]

Problem

The TUI's Model · configured view can treat an unconfigured provider as configured when the persisted provider entry contains only an empty table such as:

[providers.anthropic.http_headers]

provider_config_is_explicit previously tested whether http_headers was present, not whether it contained a meaningful nonblank entry. Empty/default provider fields could also survive serialization. A config polluted with these empty tables therefore expanded the configured picker to hundreds of global catalog rows.

This is distinct from the broader provider/catalog refactor in #3949.

Expected behavior

• Empty maps/tables and blank strings do not count as provider configuration.
• Nonempty user-authored headers, usable auth/OAuth, and meaningful model/base URL/auth fields do count.
• The active provider remains visible.
• The configured picker stays scoped while full catalog/search still discovers unconfigured providers.
• Saving config omits empty provider/header structures so pollution does not recur.

Evidence and implementation

Draft PR #4332 adds the semantic predicate, empty-structure serialization guards, existing polluted-config compatibility, and focused config/provider/picker tests. Local full TUI verification: 6,378 passed, 0 failed, 2 ignored; config crate: 368 passed, 0 failed.

Acceptance

• fix: make v0.8.68 TUI state and routing truthful #4332 is reviewed and landed.
• Empty provider headers do not appear in the configured picker after loading an existing polluted config.
• Nonempty headers and the active OpenAI Codex route remain visible.
• Unconfigured Anthropic rows remain discoverable only through full catalog/search.
• Empty provider/header tables do not survive a save round trip.

Do not close until the landed build is verified against a real previously polluted user config.

[Hmbown]

Verified against the installed v0.8.68 merge build (0.8.68 (9bfa466d64c1)) using a permission-preserving temporary copy of a real previously polluted user config; the live config was not modified and no credential values were printed.

Landed-build proof:

• Real copy contained 33 empty [providers.<name>.http_headers] tables plus an empty root [http_headers].
• In /provider configured view, Anthropic was absent; OpenAI Codex (ChatGPT) and the legitimately configured routes remained visible.
• Toggling to the full catalog made Anthropic discoverable; toggling back removed it again.
• A typed CLI ConfigStore::save round trip on the copy removed all 33 empty provider header tables and the empty root header table, preserved mode 0600, and retained active provider deepseek.
• The merged focused test non_empty_provider_headers_mark_provider_configured covers the positive nonempty-header case; full release verification passed 6,681 TUI tests and 368 config tests.

All acceptance criteria are now met on the landed and installed build.