Add backend-controlled format options to data explorer

[kv9898]

Addresses #4818. I attempted to address this problem by #7266, but that was rejected for the reason that it is more desirable to let the backends control formatting options rather than a global config. This PR takes exactly the latter approach.

Note that this PR only adds an optional format_options field to the protocol, and the functionality requires corresponding changes to the backends to respect the new protocol. posit-dev/ark#987 implements this for ark (the R backend).

The combined end result looks like (using the code example in the original issue):

Changes

Protocol Layer

• Added optional format_options field to BackendState in data_explorer-backend-openrpc.json
• Updated TypeScript and Python interfaces to include FormatOptions in backend state

Frontend Integration

• Modified DataExplorerClientInstance.updateBackendState() to merge backend-provided format options with defaults
• Added constants DEFAULT_DATA_THOUSANDS_SEP and DEFAULT_PROFILE_THOUSANDS_SEP
• Preserved distinct thousands separator handling for data display ('') vs profiles (',')

Python Backend

• Added _get_format_options() method to DataExplorerTableView base class for subclass override
• Included format_options in BackendState returned by get_state()

Documentation

• ARK_IMPLEMENTATION.md: Implementation guide for R backend with max_integral_digits = 7 + scipen calculation
• positron/comms/data_explorer.md: Protocol documentation for format options

How It Works

Backends now return format options in their state:

interface FormatOptions {
  large_num_digits: 2,
  small_num_digits: 4,
  max_integral_digits: 7 + scipen,  // Controls scientific notation threshold
  max_value_length: 1000,
  thousands_sep?: string
}

Frontend merges backend options with defaults, falling back to max_integral_digits: 7 if not provided. This allows:

• R backends to respect scipen (requires ark repository changes)
• Python backends to respect pandas settings (future)
• CSV viewer to use sensible defaults
• Full backward compatibility

Release Notes

New Features

• Data explorer now supports backend-controlled format options for numeric display. R backends can control scientific notation thresholds via format options (requires ark implementation). Addresses Data Explorer - scientific notation - does not adhere to options(scipen = xxx) #4818

Bug Fixes

• N/A

QA Notes

Risk Level: Low. Changes are additive and backward compatible.

Testing after ark implementation:

1. Test R with default scipen=0: Numbers ≥10M display as scientific notation
2. Test R with options(scipen=10): Larger numbers display in fixed notation
3. Test R with options(scipen=-3): Smaller numbers display as scientific notation
4. Verify Python and CSV viewer continue using defaults
5. Run existing data explorer test suite

Adjacent areas: None. Format option merging is self-contained in updateBackendState().

Context: The ark repository needs updates to implement format_options calculation from R's scipen option. See ARK_IMPLEMENTATION.md for detailed guidance.

[github-actions]

Thank you all for this PR! We ask that you sign our Contributor License Agreement before we accept your contribution. You can sign the CLA by posting a comment on this PR saying:

---

I have read the CLA Document and I hereby sign the CLA

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1 out of 2 committers have signed the CLA.
✅ (kv9898)[https://github.com/kv9898]
❌ @Copilot
_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}

[kv9898]

@juliasilge I can't get copilot to sign the CLA, can you help me with this?

[kv9898]

recheck

[kv9898]

@juliasilge Hmm the same problem (Copilot not included in the allowlist) also appears here:

positron/.github/workflows/cla.yml

Line 52 in 50d0337

allowlist: DavisVaughan, dependabot[bot], dfalbel, isabelizimm, jonvanausdeln, lionel-, nstrayer, petetronic, positron-bot[bot], seeM, sharon-wang, softwarenerd, timtmok, wesm, posit-jenkins-enterprise[bot], github-actions[bot]