Replace deprecated context.getFilename() in ESLint plugin - #3220

Description

eslint-plugin-graphile-export calls context.getFilename() (in
export-subclasses) and context.getPhysicalFilename() (in the
GRAPHILE_ENV=development debug output of export-methods). ESLint deprecated
both in v8.40.0 in favour of the context.filename and
context.physicalFilename properties, and
removed them in v10.0.0
(v8.40.0 release notes).
This is one of the things the plugin needs before it can support ESLint 10.

This PR adds getFilename(context) and getPhysicalFilename(context) helpers
to utils/eslint-plugin-graphile-export/src/utils.ts. They read the properties
when present and fall back to the old methods otherwise, so older ESLint
versions keep working. The three call sites use them:

• utils/eslint-plugin-graphile-export/src/ExportSubclasses.ts (2 sites)
• utils/eslint-plugin-graphile-export/src/ExportMethods.ts (1 site)

The peer range is unchanged. A patch changeset is included.

Performance impact

None.

Security impact

None.

Verification

Run in a node:22-bookworm container after yarn --immutable and
yarn build. Each check was run on main first, then again with this change.
Both runs gave the same results:

• yarn tsc -b in utils/eslint-plugin-graphile-export: passes.
• eslint --ext .js,.jsx,.ts,.tsx utils grafast/grafast (the repo config,
  which enables the plugin's recommended rules): exit 0, no reports. The
  full yarn eslint . needs more memory than the container had.
• A throwaway script that runs both rules through ESLint's Linter on
  class MyStep extends Step {} and { plan() {}, notAllowed: 1 } in
  /work/src/MyStep.ts. The "convert to importable" suggestion contains
  "./MyStep.ts", and the development debug message contains
  /work/src/MyStep.ts:3:13.
• yarn prettier --check on the changed files: passes.

After the change to the helpers (ea94be7), re-run in the same container:

• yarn tsc -b in utils/eslint-plugin-graphile-export: passes.
• yarn eslint --ext .js,.jsx,.ts,.tsx utils/eslint-plugin-graphile-export:
  exit 0. The wider utils grafast/grafast run now exits 129 with no output on
  both main and this branch, so it is not counted here.
• The throwaway script above: same results.
• The same script with a rule context where filename and physicalFilename
  are undefined and only getFilename() / getPhysicalFilename() exist (as
  on ESLint < 8.40): same results, with the fallback methods called for all
  three sites.
• yarn prettier --check on the changed files: passes.

Checklist

• My code matches the project's code style and yarn lint:fix passes. (ESLint and Prettier were run on the scope above, not the full lint:fix.)
• I've added tests for the new feature, and yarn test passes. (The plugin has no test suite. It was checked with the script above.)
• I have detailed the new feature in the relevant documentation. (Not applicable.)
• I have added this feature to 'Pending' in the RELEASE_NOTES.md file (if one exists). (A changeset is included instead.)
• If this is a breaking change I've explained why. (Not a breaking change.)

Found with unstale, a tool I'm building so nobody has to read every changelog to catch changes like this one. Prepared with AI assistance (Claude Code) and verified as described above.