fix(git): filter git show <rev>:<path> blob reads past a size threshold

[albatrossflyon-coder]

Fixes #3116

Root cause

git show <rev>:<path> (blob read form) was routed to raw passthrough unconditionally in run_show, same as --stat/--pretty output. Unlike the diff form of git show, which auto-compacts below in the same function, the blob form never went through any filtering — confirmed against a real 5-week command-history DB: 436 calls, 3.82M input tokens, 0.6% aggregate savings vs. 50%+ for the <rev> -- <path> diff form.

Fix

• Blobs at or under 200 lines (BLOB_FILTER_LINE_THRESHOLD) stay byte-identical passthrough — not worth a filter pass, per the issue's own suggested conservative threshold.
• Past that, the blob is run through the existing MinimalFilter (the same comment/blank-line-stripping filter rtk read -l minimal uses), with the language detected from the path segment of the rev:path arg.
• never_worse (already used everywhere else in this file) guards the result — if filtering doesn't actually shrink the output, raw is shown instead.
• Tracking label switches to (blob-filtered) vs (passthrough) depending on whether filtering actually reduced size, so telemetry can distinguish the two going forward.

Verification

• cargo test — 2474 passed, 0 failed (3 new tests added: under-threshold passthrough stays byte-identical, over-threshold strips comments while preserving code, unknown-extension blobs are still bounded by never_worse).
• cargo fmt --check / cargo clippy -- -D warnings — clean.
• Live repro against this repo's own git.rs (3367 lines): raw git show HEAD:src/cmds/git/git.rs is 113,289 bytes; rtk git show of the same blob is 105,016 bytes (comments/blank-lines stripped, code untouched — diffed to confirm no code lines were removed, only // comment lines).
• Small file (Cargo.toml, well under the threshold) is confirmed byte-identical between git show and rtk git show.

The 200-line threshold is a first cut per the issue's own suggestion ("so tiny files stay passthrough") — happy to tune it if you'd rather key off byte size or make it configurable.

[KuSh]

Thanks for working on this. Blob reads with rtk git show <rev>:<path> are now filtered on develop by #3265 (merged 2026-09-11, shipped in v0.50.0): large blobs get an 8 KiB preview with a hint pointing at the rest, and #3116 was closed on that basis. Small blobs already pass through unchanged there, as this PR proposed. If a case is still missed, please comment here or reopen.