feat(zeph-tools,zeph-llm): add ToolOutput.media plumbing for MCP image passthrough

[bug-ops]

Summary

• Foundational, behavior-preserving plumbing for multimodal MCP ContentBlock::Image passthrough (spec-072). This is P0 of a 4-phase implementation (P0 plumbing -> P1 persistence strip -> P2 decode/validate/attach -> P3 config/CLI/TUI surface); see follow-up issues for P1-P3.
• Adds ToolOutput.media: Vec<zeph_llm::ImageData> (empty by default) with #[derive(Default)] on ToolOutput, a new zeph-tools -> zeph-llm crate dependency edge, and a hand-written redacting impl Debug for ImageData ([image: {mime}, {n} bytes], never the raw bytes) hardening a latent Debug-derive leak on the existing user-upload image path.
• Migrates 130 ToolOutput { .. } construction sites across the workspace to ..Default::default() so the new field doesn't require touching every call site's logic.
• media is never populated in this PR, so it changes no runtime behavior. Decode/validate/attach logic, the persistence strip (SQLite/Qdrant/durable JSONL), vision-tier routing, and the config/CLI/TUI opt-in surface are deferred to P1-P3.

Part of #6229 (P0 only — this PR does NOT close the issue; the issue tracks the full feature and stays open until P1-P3 land as follow-up issues referencing specs/072-multimodal-mcp-passthrough/plan.md).

Test plan

• cargo +nightly fmt --check
• cargo clippy --profile ci --workspace --all-targets --features "desktop,ide,server,chat,pdf,scheduler,testing" -- -D warnings
• cargo clippy --profile ci --workspace --all-targets --features bench -- -D warnings
• cargo nextest run --config-file .github/nextest.toml --workspace --features "desktop,ide,server,chat,pdf,scheduler" --lib --bins (13475/13475 passed)
• cargo test --doc --workspace --features "desktop,ide,server,chat,pdf,scheduler"
• RUSTFLAGS="-D warnings" RUSTDOCFLAGS="--deny rustdoc::broken_intra_doc_links" cargo doc --no-deps --workspace --features "desktop,ide,server,chat,pdf,scheduler"
• gitleaks protect --staged
• New unit tests: test_tool_output_default_media_empty, test_image_data_debug_redacts_bytes (asserts exact redacted output and absence of the raw byte values)
• No LLM Serialization Gate live-session test required for this PR — zero new MessagePart::Image emission or provider request-builder changes (P2 scope)