Add official Firebird dialect support

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Feature Description
Describe the feature you'd like to see implemented
Official support for Firebird (https://firebirdsql.org/) as a Sequelize v7 dialect, following the same AbstractDialect structure as the existing dialects (postgres, mysql, sqlite3, mssql, db2, ibmi, ...).

I've already built this as a standalone package, verified against real Firebird servers (not just unit tests):

• Repo: https://github.com/MahmoudTL/sequelize-firebird
• CI: green across Firebird 2.5, 3.0, and 5.0 (Docker-based, see ci.yml at https://github.com/MahmoudTL/sequelize-firebird/blob/master/.github/workflows/ci.yml); Firebird 2.1 verified locally. Firebird 4.0 is the only version I haven't been able to verify, for lack of a working Docker image (tracked in ROADMAP.md at https://github.com/MahmoudTL/sequelize-firebird/blob/master/ROADMAP.md)
• Verified end-to-end: connection/auth, raw queries, Model.sync(), full CRUD via RETURNING, bulk operations, associations (hasMany/belongsTo, nested include, FK constraints), migrations (addColumn/changeColumn/renameColumn/removeColumn, describeTable, showConstraints), transactions, and savepoints (nested transactions)
• Along the way, found and fixed about 15 real bugs specific to Firebird's quirks (no multi-row INSERT ... VALUES, no native AUTO_INCREMENT before v3, no SQL-text START TRANSACTION/COMMIT/ROLLBACK, 31-byte identifier limit before v4, RETURNING only works on single-row statements, etc.) - all documented in the roadmap
  Usage example:

import { Sequelize } from '@sequelize/core'; import { FirebirdDialect } from 'sequelize-firebird'; // -> '@sequelize/firebird' if adopted

const sequelize = new Sequelize({ dialect: FirebirdDialect, host: 'localhost', port: 3050, database: 'employee', user: 'sysdba', password: 'masterkey', });

Describe why you would like this feature to be added to Sequelize
Firebird still has an active user base (embedded/small-footprint deployments, legacy systems in some industries and regions), but has no first-class Sequelize support today. I originally asked about this in discussion #18249 (https://github.com/sequelize/sequelize/discussions/18249) and got positive, detailed feedback - though on reflection I should flag that the two replies I got there were from a community member with no maintainer affiliation (author_association: NONE), not from the Sequelize team, so I don't actually have a maintainer's read on this yet. That's what I'm hoping to get here.

I understand a new dialect is a real long-term maintenance commitment, and I'm not assuming this should land in the core monorepo immediately - happy to keep it as a well-maintained standalone package (following AbstractDialect exactly, so it could be absorbed later) if that's preferred. Mainly looking for a maintainer's take on whether there's a path to official support, and what the bar for that would look like concretely (unit test coverage matching the other dialects' ~106-file suites is the obvious big remaining gap).

@ephys, given your work on the v7 dialect architecture, I'd especially value your read on whether this is worth pursuing further, if you have the bandwidth.

Is this feature dialect-specific?
Yes. This feature only applies to the following dialect(s): Firebird (new dialect)

Would you be willing to resolve this issue by submitting a Pull Request?
Yes, I have the time but I will need guidance.

Start by reviewing the standalone sequelize-firebird package and its AbstractDialect implementation, then compare it with Sequelize's existing dialect structures. Check ci.yml and ROADMAP.md for supported Firebird versions and known gaps, and review the stated need for a roughly 106-file unit-test suite. Done requires maintainer guidance on whether official support belongs in core and what acceptance bar applies.