Survive TML spec drift in deploy and flag unparseable checkpoint exports

[jp-spotter]

TS Cloud 26.7 added viz_style to the TML spec; no released thoughtspot_tml parses it (thoughtspot/thoughtspot_tml#40, same drift class as #29). Export writes raw edoc text and succeeds, so checkpoints from 26.5+ clusters produce files that crash every subsequent deploy with an unhandled TMLDecodeError before anything is sent.

Deploy now survives spec drift

• Per-file parse failures are collected instead of raised. ALL_OR_NONE refuses up front (a partial set violates the policy; exit 1). PARTIAL / VALIDATE_ONLY proceed with the parseable set, report the skips, and exit 1.
• When drift is the cause, the failure reason names the real fix: uses TML attributes newer than the installed thoughtspot_tml <version> (unrecognized attribute: 'viz_style').

Checkpoint flags the landmines at export time

• Exports stay raw-text (never parse-then-dump — unknown attributes are never stripped). After writing, each edoc is parse-checked; unparseable files get a WARN naming object + attribute, plus a count summary. Files are kept and the exit code is unchanged.

Reporting is capped — first 5 failures in detail, then ..and N more, full list in the logfile. Live verification against a 26.7 cluster surfaced 223 drifted Liveboards out of 2,341.

Also: python -m cs_tools now exits with the command's return code — __main__.py called run() without sys.exit(), so module-mode invocations always exited 0 regardless of failure. The console-script exe was never affected. Adds the suite's first process-level exit-code test (verified red before the fix).

Verified live against 26.7: landmine detection on genuine drift, VALIDATE_ONLY partial deploy + skip reporting (exit 1), ALL_OR_NONE pre-flight refusal before any API call (exit 1).

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Side note: a fix in thoughtspot_tml itself is also in motion (spec regen against 26.7.x, tracked upstream in thoughtspot/thoughtspot_tml#40). Once a fixed release ships we'll bump the pyproject.toml floor in a follow-up — the hardening here covers the drift class regardless, including the next attribute the server adds.