fix: isolate unrenderable glyph failures

[oz-for-oss]

Closes #9372

Summary

• Cache deterministic glyph rasterization failures as missing glyphs so one unsupported fallback glyph cannot blank the entire terminal glyph layer.
• Continue rendering remaining glyphs in WGPU and Metal renderers when a per-glyph cache error occurs.
• Return a controlled swash rasterizer error instead of panicking when a subpixel raster image is unavailable.
• Add glyph cache unit tests covering raster-bounds failures, rasterization failures, and rendering other glyphs after a cached missing glyph.

Validation

• cargo test --manifest-path /workspace/warp/Cargo.toml -p warpui rendering::glyph_cache --lib
• cargo check --manifest-path /workspace/warp/Cargo.toml -p warpui --lib

Notes

• cargo fmt --manifest-path /workspace/warp/Cargo.toml --all could not run because rustfmt is not installed for the stable toolchain in this sandbox.